INTRODUCTION
Since the 1980s, surgical pathologists in general and infectious disease pathologists
in particular have dealt with an increasing number of surgical specimens from patients
in whom one or multiple infectious agents may be responsible for disease.
1
In this context, pathologists have played an important role in recognizing infectious
agents. In many cases, when fresh tissue is not available for culture, pathologists
can provide a rapid morphologic diagnosis and facilitate clinical decisions in patient
treatment.
2
In addition, pathologists have played a central role in the identification of emerging
and reemerging infectious agents and describing the pathogenetic processes of emerging
diseases, such as hantavirus pulmonary syndrome and other viral hemorrhagic fevers,
leptospirosis, and rickettsial and ehrlichial infections as well as the diagnosis
of anthrax during the bioterrorist attack of 2001.3, 4, 5, 6, 7
Conventionally, microbial identification in infectious diseases has been made primarily
by using serologic assays and culture techniques. However, serologic results can be
difficult to interpret in the setting of immunosuppression or when only a single sample
is available for evaluation. In addition, fresh tissue is not always available for
culture, and culture of fastidious pathogens can be difficult and may take weeks or
months to yield results. Moreover, culture alone cannot distinguish colonization from
tissue invasion. Some microorganisms have distinctive morphologic characteristics
that allow their identification in formalinfixed tissues using routine and special
stains. Nevertheless, in several instances it is difficult or even impossible to identify
an infectious agent specifically by conventional morphologic methods.
Immunohistochemistry is one of the most powerful techniques in surgical pathology.
There has been an increasing interest in the use of specific antibodies to viral,
bacterial, fungal and parasitic antigens in the detection and identification of the
causative agents in many infectious diseases. The use of a specific antibody to detect
a microbial antigen was first performed by Coons and associates
8
to detect pneumococcal antigen in tissues. The advantages of immunohistochemistry
over conventional staining methods (Table 2.1
) and the contributions of immunohistochemistry in infectious diseases (Table 2.2
) are substantial. It is important to emphasize that both monoclonal and polyclonal
antibodies must be tested for possible cross-reactivities with other organisms. The
widespread occurrence of common antigens among bacteria and pathogenic fungi is well
established.
1,
9
Finally it is important to understand that immunohistochemistry has several steps
and all of them can affect the final result; however, in general the only limitations
are the availability of specific antibodies and the preservation of epitopes.
1,
10
It is well known that for larger microorganisms such as protozoa, fungi and some bacteria,
pretreatment of formalinfixed, paraffin-embedded tissue is not required. In contrast,
for smaller infectious agents, for example, microorganisms such as viruses and chlamydiae,
pretreatment of the tissue with proteolytic enzymes or heat-induced epitope retrieval
is necessary in order to enhance immunoreactivity.
Table 2.1
Advantages of IHC for the diagnosis of infectious diseases
Allows rapid results
Can be performed on formalinfixed, paraffin-embedded tissue, reducing the risk of
exposure to serious infectious diseases
High sensitivity allowing identification of infectious agents even before morphologic
changes occur
Useful for retrospective diagnosis of individual patients and for in-depth study of
the disease
Specificity: monoclonal antibodies and some polyclonal antibodies allow for specific
identification of infectious agents
Table 2.2
Contributions of IHC to the diagnosis of infectious diseases
Allows identification of new human pathogens
Allows microbiologic/morphologic correlation establishing the pathogenic significance
of microbiological results
Provides a rapid morphologic diagnosis allowing early treatment of serious infectious
diseases
Contributes to understanding of the pathogenesis of infectious diseases
Provides a diagnosis when fresh tissue is not available or when culture methods do
not exist
Table 2.3
lists currently available antibodies for diagnostic use in surgical pathology.
Table 2.3
Some commercially available antibodies for immunohistochemical diagnosis of infectious
diseases
Microorganism
Antibody/clone
Dilution
Pretreatment
Source
Adenovirus
Mab/20/11 and 2/6
1:2000
Proteinase K
Chemicon
Aspergillus
Mab/WF-AF-1
1:200
HIAR*
Dako
B. henselae
Mab
1:100
HIAR
Biocare Medical
BK virus
Mab/BK T. 1
1:8000
Trypsin
Chemicon
C. albicans
Mab/1B12
1:400
HIAR
Chemicon
C. pneumoniae
Mab/RR402
1:200
HIAR
Accurate
Cryptosporidium
Mab/Mabc1
1:100
HIAR
Novocastra
CMV
Mab/DDG9/CCH2
1:50
HIAR
Novocastra
Giardia intestinalis
Mab/9D5.3.1
1:50
HIAR
Novocastra
Hepatitis B core Ag.
Rabbit polyclonal
1:2000
HIAR
Dako
Hepatitis B surface Ag.
Mab/3E7
1:100
HIAR
Dako
Herpes simplex 1 and 2
Rabbit polyclonal
1:3200
HIAR
Dako
H. pylori
Rabbit polyclonal
1:40
Protase 1
Dako
HHV 8
Mab/LNA-1
1:500
HIAR
Novocastra
L. monocytogenes
Rabbit polyclonal
1:5000
Proteinase K
Difco
Parvovirus B19
Mab/R92F6
1:500
HIAR
Novocastra
P. carinii
Mab/3F6
1:20
HIAR
Novocastra
Respiratory syncytial virus
Mab/5H5N
1:200
HIAR
Novocastra
T. gondii
Rabbit polyclonal
1:320
HIAR
Biogenex
West Nile virus
Mab/5H10
1:400
Proteinase K
Bioreliance
*
Heat-induced antigen retrieval.
VIRAL INFECTIONS
Immunohistochemistry has played an important role not only in the diagnosis of a large
number of viral infections but also in the study of their pathogenesis and epidemiology.
Traditionally, the diagnosis of viral infections has relied on cytopathic changes
observed on routine histopathology. Several viral pathogens produce characteristic
intracellular inclusions, which allow pathologists to make a presumptive diagnosis
of viral infection. However, for some viral infections the characteristic cytopathic
changes are often subtle and sparse, requiring a meticulous search.
11
Moreover, only 50% of the known viral diseases are associated with characteristic
intracellular inclusions.
12
In addition, formalin, which is the most commonly used fixative in histopathology,
is a poor fixative for demonstrating the morphologic and tinctorial features of viral
inclusions.
12
When viral inclusions are not detected in hematoxylin-eosin stained sections, or when
the viral inclusions present cannot be differentiated from those of other viral diseases,
immunohistochemical techniques offer a more reliable alternative to reach a specific
diagnosis.
Hepatitis B virus
Hepatitis B virus infection constitutes an important cause of chronic hepatitis in
a significant proportion of patients. In many instances, the morphologic changes induced
by hepatitis B virus on hepatocytes are not typical enough to render a presumptive
diagnosis of hepatitis B viral infection. In other instances, there may be so little
hepatitis B surface antigen (HBsAg) that it cannot be demonstrated by techniques such
as orcein staining. In these cases, immunohistochemical techniques to detect HBsAg
are more sensitive than histochemical methods and are helpful in reaching the diagnosis.
13
Immunostaining for HBsAg has been used in the diagnosis of hepatitis B and in the
study of carrier states.
14,
15
Eighty percent or more of cases with positive serologic results for HBsAg demonstrate
cytoplasmic HBsAg using immunohistochemistry (Fig. 2.1
).
16
By immunoperoxidase localization, hepatitis B core antigen (HBcAg) can be demonstrated
within the nuclei or the cytoplasm of hepatocytes, or both (Fig. 2.2
). Predominantly cytoplasmic expression of HBcAg is associated with a higher grade
of hepatitis activity.
17
Fig. 2.1
Liver biopsy specimen from a patient with chronic hepatitis B. Scattered hepatocytes
show cytoplasmic reactivity with monoclonal antibody to HBsAg. (Immunoperoxidase staining
with diaminobenzidine [DAB] and hematoxylin counterstain, ×400)
Fig. 2.2
Chronic active hepatitis B. Numerous hepatocytes display intranuclear reactivity with
polyclonal antibody to hepatitis B core antigen (HBcAg). (Immunoperoxidase staining
with DAB and hematoxylin counterstain, ×400)
Hepatitis C virus
The clinical diagnosis of hepatitis C virus (HCV) infection is based on serological
demonstration of antibodies against HCV and detection of HCV RNA in serum. However,
anti-HCV antibodies may be not detectable in sera of immunocompromised patients.
18
Several polyclonal and monoclonal antibodies directed against HCV nonstructural proteins
have been produced for use in immunohistochemistry. Nevertheless, most of the antibodies
are not clinically useful because of low sensitivities compared with HCV RNA detection
by RT-PCR.18, 19, 20, 21 Moreover, cross-reactivity with non-HCV epitopes has been
found with the monoclonal antibody TORDJI-22.
21,
22
Diffuse or coarse granular cytoplasmic staining is usually seen in a variable number
of hepatocytes in patients with chronic HCV hepatitis,
19,
23,
24
and within rare biliary epithelial cells, lymphocytes, and sinusoidal endothelial
cells.
More recently, a monoclonal antibody against HCV E2 envelope glycoprotein has been
demonstrated to be a highly sensitive antibody for the diagnosis and clinical follow-up
of chronic HCV hepatitis with an overall accuracy of 95% when used with the EnVision
technique.
25
This antibody is useful for the early detection of graft reinfection in patients with
liver transplant for HCV-related cirrhosis, and to differentiate reinfection from
graft rejection.
25
Herpesviruses
Histologically, the diagnosis of herpes simplex virus (HSV) infection involves the
detection of multinucleated giant cells containing characteristic molded, ground glass-appearing
nuclei and Cowdry's type A intranuclear inclusions. When there are abundant viral
inclusions within infected cells, the diagnosis is usually straightforward. However,
the diagnosis of HSV infection can be difficult when the characteristic intranuclear
inclusions or multinucleated cells, or both, are absent or when the amount of tissue
in a biopsy specimen is small.
26
In these cases, the use of immunohistochemistry to detect HSV antigens is advantageous.
27,
28
Immunohistochemistry using either polyclonal or monoclonal antibodies against HSV
antigens has proven to be a sensitive and specific technique to diagnose HSV infections
(Fig. 2.3
).
29,
30
Although polyclonal antibodies against the major HSV glycoprotein antigens are sensitive,
they do not allow distinction between HSV-1 and HSV-2 because these two viruses are
antigenically similar.
31
In addition, the histologic features of HSV infection are not specific and can also
occur in patients with varicella-zoster (VZV) infection. Monoclonal antibodies against
the VZV envelope glycoprotein gp1 are sufficiently sensitive and specific to allow
a clear-cut distinction between HSV and VZV infections.
27,
32,
33
Fig. 2.3
Herpes simplex hepatitis. The nuclei and cytoplasm of many hepatocytes and Kupffer
cells are strongly immunostained for herpes simplex antigen. (Immunoperoxidase staining
with DAB and hematoxylin counterstain, ×400)
Immunohistochemistry has also been useful in demonstrating the association of human
herpesvirus 8 (HHV-8) with Kaposi's sarcoma, primary effusion lymphoma, and multicentric
Castleman's disease.34, 35, 36, 37, 38 The diagnosis of Kaposi's sarcoma may be problematic
due to its broad morphologic spectrum and similar appearance to other benign and malignant
neoplastic vascular lesions. Immunostaining of Kaposi's sarcoma latent associated
nuclear antigen-1 (LANA-1) is useful to confirm the diagnosis of Kaposi's sarcoma,
particularly in difficult early lesions or when the neoplasm presents in an unusual
location, and allows distinction of Kaposi's sarcoma from several morphologically
similar vasoproliferative lesions (see Chapter 12).
39,
40
Immunostaining is restricted to the nuclei of spindle cells and endothelial cells
of the slit-like vascular spaces. Immunohistochemistry has also demonstrated expression
of HHV-8 LANA-1 in mesothelial cells of HIV-associated recurrent pleural effusions
41
and in the cells of the plexiform lesions of primary pulmonary hypertension.
42
Cytomegalovirus (CMV) is an important opportunistic pathogen in immunocompromised
patients. Histologic diagnosis of CMV in fixed tissues usually rests on the identification
of characteristic cytopathic effects, including intranuclear or cytoplasmic inclusions,
or both. However, histologic examination lacks sensitivity, and in some cases atypical
cytopathic features can be confused with reactive or degenerative changes.
43
In these cases, immunohistochemistry using monoclonal antibodies against early and
late CMV antigens allows the detection of CMV antigens in the nucleus and cytoplasm
of infected cells (Fig. 2.4
). In addition, immunohistochemistry may allow detection of CMV antigens early in
the course of the disease when cytopathic changes have not yet developed.44, 45, 46,
47, 48, 49 For example, CMV early nuclear antigen is expressed 9 to 96 hours after
cellular infection and indicates early active viral replication. Immunohistochemistry
has been useful in the detection of CMV infection in patients with steroid refractory
ulcerative colitis, and in detecting occult CMV infection of the central nervous system
in liver transplant patients who develop neurological complications.
50,
51
It has also been used to demonstrate a high frequency of CMV antigens in tissues from
first trimester abortions.
52
The sensitivity of immunohistochemistry is better than light microscopic identification
of viral inclusions and compares favorably with culture and in situ hybridization.44,
46, 47, 49, 53 Additionally, immunohistochemical assays can be completed faster than
the shell vial technique, with immunofluorescence, or culture allowing for rapid results
that are important for early anti-CMV therapy.
49
Fig. 2.4
Cytomegalovirus (CMV) villitis in a case of congenital CMV infection. Stromal cells
and Hofbauer cells show intranuclear and cytoplasmic CMV antigen. (Immunoperoxidase
staining with diaminobenzidine [DAB] and hematoxylin counterstain, ×400)
Adenoviruses
Adenovirus is increasingly recognized as a cause of morbidity and mortality among
immunocompromised patients owing to transplant and congenital immunodeficiency.
54,
55
Rarely, adenovirus infection has been described in HIV-infected patients.56, 57, 58
Characteristic adenovirus inclusions are amphophilic, intranuclear, homogeneous, and
glassy. However, in some cases, the infection may contain only rare cells showing
the characteristic cytopathic effect.
57
In addition, other viral inclusions, including CMV, human papillomavirus, HSV, and
VZV, can be mistaken for adenovirus inclusions and vice versa. Moreover, in immunosuppressed
patients the incidence of coinfection with other viruses is high. In these circumstances
immunohistochemical assay may be necessary for a definitive diagnosis. A monoclonal
antibody that is reactive with all 41 serotypes of adenovirus has been used in an
immunohistochemical technique to demonstrate intranuclear adenoviral antigen in immunocompromised
patients (Fig. 2.5
).57, 58, 59, 60, 61 Histologic diagnosis of adenovirus colitis is difficult, and
it is usually underdiagnosed. Immunohistochemical staining has been of value in differentiating
adenovirus colitis from CMV colitis.
57,
62
Fig. 2.5
Adenovirus pneumonia. Infected cells within a necrotizing exudate show intranuclear
reactivity with antibody to adenovirus antigen. (Immunoperoxidase staining with aminoethylcarbazole
[AEC] and hematoxylin counterstain, ×400)
Other herpesviruses infections that have been diagnosed using immunohistochemical
methods include Epstein-Barr viral infection
63
and human herpesvirusus 6 infection.
64
Parvovirus B19
Parvovirus B19 has been associated with asymptomatic infections, erythema infectiosum,
acute arthropathy, aplastic crisis, hydrops fetalis, and chronic anemia and red cell
aplasia. The diagnosis of parvovirus infection can be achieved by identifying typical
findings in bone marrow specimens, including decreased or absent red cell precursors,
giant pronormoblasts, and eosinophilic or amphophilic intranuclear inclusions in erythroid
cells.
65,
66
Because intravenous immunoglobulin therapy is effective, a rapid and accurate diagnostic
method is important. Immunohistochemistry with a monoclonal antibody against VP1 and
VP2 capsid proteins has been used as a rapid and sensitive method to establish the
diagnosis of parvovirus B19 infection in formalinfixed, paraffin-embedded tissues.67,
68, 69, 70 Immunohistochemistry is of particular help in detecting parvovirus B19
antigen in cases with sparse inclusions, to study cases not initially identified by
examination of routinely stained tissue sections, or in cases of hydrops fetalis where
there is advanced cytolysis (Fig. 2.6
).
67,
71,
72
Several studies have found a good correlation between morphologic, immunohistochemical,
in situ hybridization and polymerase chain reaction (PCR).
66,
67,
70,
72
Fig. 2.6
Hydrops fetalis caused by parvovirus B19 infection. Normoblasts within the villous
capillaries show intranuclear viral antigen. (Immunoperoxidase staining with DAB and
hematoxylin counterstain, ×600)
Viral hemorrhagic fevers
Since the 1980s, numerous emerging and reemerging agents of viral hemorrhagic fevers
have attracted the attention of pathologists.3, 4, 5 These investigators have played
an important role in the identification of these agents and supporting epidemiologic,
clinical, and pathogenetic studies of the emerging viral hemorrhagic fevers.
4,
5,
7
Viral hemorrhagic fevers are often fatal, and in the absence of bleeding or organ
manifestations these diseases are clinically difficult to diagnose and frequently
require handling and testing of potentially dangerous biological specimens. In addition,
histopathologic features are not pathognomonic, and they can resemble other viral,
rickettsial and bacterial (e.g., leptospirosis) infections. Immunohistochemistry is
essential and has been successfully and safely applied to the diagnosis and study
of the pathogenesis of these diseases.
Several studies have established the utility of immunohistochemistry as a sensitive,
safe, and rapid diagnostic method for the diagnosis of viral hemorrhagic fevers such
as yellow fever (Fig. 2.7
),73, 74, 75 dengue hemorrhagic fever,
75,
76
Crimean-Congo hemorrhagic fever,
77
Argentine hemorrhagic fever,
78
Venezuelan hemorrhagic fever,
79
and Marburg disease.
80
Additionally, a sensitive, specific, and safe immunostaining method has been developed
to diagnose Ebola hemorrhagic fever in formalinfixed skin biopsies (Fig. 2.8
).
81
Immunohistochemistry demonstrated that Lassa virus targets primarily endothelial cells,
mononuclear inflammatory cells, and hepatocytes (Fig. 2.9
).81, 82, 83
Fig. 2.7
Yellow fever. Abundant yellow fever viral antigen is seen within hepatocytes and Kupffer
cells. (Immunoperoxidase staining with AEC and hematoxylin counterstain, ×400)
(Courtesy of Dr. JF Aronson, University of Texas Medical Branch.)
Fig. 2.8
Ebola virus. Extensive Ebola viral antigens are seen primarily within fibroblasts
in dermis of a skin specimen from a fatal case of Ebola hemorrhagic fever. (Immunoalkaline
phosphatase with naphthol fast red substrate and hematoxylin counterstain, original
magnification ×20)
Fig. 2.9
Lassa fever. Liver from a patient with Lassa fever. Scattered hepatocytes and reticuloendothelial
cells show reactivity with monoclonal antibody to Lassa virus. (Naphthol fast red
substrate and hematoxylin counterstain, original magnification ×100)
Papovaviruses
Immunohistochemistry for the detection of human papillomavirus in formalinfixed tissue
has been replaced by more sensitive diagnostic molecular techniques such in situ hybridization.84,
85, 86, 87 In addition to low sensitivity compared with molecular techniques, immunohistochemistry
detects only productive and not latent infections and cannot be used to determine
the type of virus present (Fig. 2.10
).
Fig. 2.10
Immunoperoxidase staining for human papillomavirus (HPV) in a patient with mild squamous
dysplasia. HPV viral antigen localizes within the nuclei of koilocytotic cells. (DAB
with hematoxylin counterstain, ×600)
BK virus infections are frequent during infancy; in immunocompetent individuals the
virus remains latent in the kidneys, central nervous system and B-lymphocytes. In
immunocompromised patients, the infection reactivates and spreads to other organs.
In the kidney, the infection is associated with mononuclear interstitial inflammatory
infiltrates and tubular atrophy, findings that can be difficult to distinguish from
acute transplant rejection. Besides, the cytopathic changes observed in BK virus infection
are not pathognomonic and can be observed in other viral infections. In this setting,
immunohistochemistry has been useful to demonstrate BK virus infection.88, 89, 90,
91
The human polyomavirus JC is a double-stranded DNA virus that causes progressive multifocal
leukoencephalopathy (PML). This fatal demyelinating disease is characterized by cytopathic
changes in oligodendrocytes and bizarre giant astrocytes. Immunohistochemical technique
using a polyclonal rabbit antiserum against the protein VP1 is a specific, sensitive,
and rapid method for confirming the diagnosis of PML.92, 93, 94, 95 JC virus antigen
is usually seen within oligodendrocytes and occasional astrocytes, and antigen-bearing
cells are more commonly seen in early lesions.
Other viruses
Immunohistochemistry has also been used to confirm the diagnosis of respiratory viral
diseases such as influenza A virus and respiratory syncytial virus infections when
cultures were not available.96, 97, 98, 99
The diagnosis of rabies relies heavily on histopathologic examination of tissues to
demonstrate the characteristic cytoplasmic inclusions (Negri bodies). In an important
percentage of cases, Negri bodies may be inconspicuous and so few that confirming
the diagnosis of rabies may be extremely difficult.
100
Furthermore, in nonendemic areas the diagnosis of rabies is usually not suspected
clinically or the patient can present with an ascending type of paralysis. In these
settings, immunohistochemical staining is a very sensitive, safe, and specific diagnostic
tool for rabies (Fig. 2.11
).100, 101, 102, 103, 104 Other viral agents that can be diagnosed using immunohistochemical
methods include enterovirus,105, 106, 107, 108 Eastern equine encephalitis,109, 110,
111 and rotavirus.112, 113, 114
Fig. 2.11
Rabies. Immunostaining of rabies viral antigens in neurons of CNS using a rabbit polyclonal
antibody. Red precipitate corresponds to Negri inclusions on H&E. (Immunoalkaline
phosphatase with naphthol fast red substrate and hematoxylin counterstain, original
magnification ×40)
BACTERIAL INFECTIONS
Among bacterial infections, the greatest number of immunohistochemical studies have
been performed in the investigation of Helicobacter pylori. A few studies have evaluated
the use of immunohistochemistry in other bacterial, mycobacterial, rickettsial, and
spirochetal infections.
Antigen retrieval is generally not required for the immunohistochemical demonstration
of bacteria in fixed tissue. However, interpretation of the results can be complicated
by the fact that many of these antibodies will cross-react with other bacteria. Moreover,
antibodies may react with only portions of the bacteria, and they may label remnants
of bacteria or spirochetes when viable organisms are no longer present.
Helicobacter pylori infection
Gastric infection by H. pylori results in chronic active gastritis and is strongly
associated with lymphoid hyperplasia, gastric lymphomas, and gastric adeno-carcinoma.
Heavy infections with numerous organisms are easily detected on routine hematoxylin
and eosin-stained tissues; however, the detection rate is only 66% with many false-positive
and false-negative results.115, 116, 117 Conventional histochemical methods such as
silver stains are more sensitive than hematoxylin and eosin in detecting H. pylori.
Nonetheless, for the detection of scant numbers of organisms, immunohistochemistry
has proved to be highly specific and sensitive, less expensive when all factors are
considered, and superior to conventional histochemical methods (Fig. 2.12
).
116,
117
Treatment for chronic active gastritis and H. pylori infection can change the shape
of the microorganism, making difficult its identification and differentiation from
extracellular debris or mucin globules. In these cases, immunohistochemistry improves
the rate of successful identification of the bacteria even when histologic examination
and cultures are falsely negative.118, 119, 120, 121
Fig. 2.12
Numerous curved Helicobacter pylori in the superficial mucus are clearly demonstrated
by immunoperoxidase staining in this patient with chronic active gastritis. (DAB with
hematoxylin counterstain, ×600)
Whipple's disease
Whipple's disease affects primarily the small bowel and mesenteric lymph nodes and
less commonly other organs such as heart and central nervous system. Numerous foamy
macrophages characterize the disease, and the diagnosis usually relies on the demonstration
of PAS-positive intracytoplasmic bacteria. Nevertheless, the presence of PAS-positive
macrophages is not pathognomonic; they can be observed in other diseases such as Mycobacterium
avium complex infections, histoplasmosis, infections due to Rhodococcus equi, and
macroglobulinemia. Immunohistochemical staining with a rabbit polyclonal antibody
provides a sensitive and specific method for the rapid diagnosis of intestinal and
extraintestinal Whipple's disease and for follow-up of treatment response.122, 123,
124
Rocky Mountain spotted fever
Confirmation of Rocky Mountain spotted fever (RMSF) usually requires the use of serologic
methods to detect antibodies to spotted fever group (SFG) rickettsiae; yet a significant
percentage of patients with RMSF lack diagnostic titers during the first week of disease.
Immunohistochemistry has been successfully used to detect SFG rickettsiae in formalinfixed
tissue sections (Fig. 2.13
).
125,
126
Several studies have illustrated the value of immunohistochemistry in the diagnosis
of suspected cases of RMSF using skin biopsies, and in confirming fatal cases of seronegative
RMSF.
127,
128
Fig. 2.13
Immunohistologic demonstration of Rickettsia rickettsii within endothelial cells surrounded
by a small glial nodule in the brainstem of this patient with fatal Rocky Mountain
spotted fever. (Immunoperoxidase staining with AEC and hematoxylin counterstain, ×600)
Bartonella infections
Bartonella are slow growing, fastidious Gram-negative, Warthin-Starry-stained bacteria
associated with bacillary angiomatosis, peliosis hepatis, cat-scratch disease, and
blood culture-negative endocarditis. Immunostaining has been successfully used to
identify Bartonella henselae and B. quintana in heart valves from patients with blood
culture-negative endocarditis (Fig. 2.14
).
129,
130
This polyclonal rabbit antibody that does not allow differentiation between B. henselae
and B. quintana has also been used in the detection of these microorganisms in cat-scratch
disease, bacillary angiomatosis, and peliosis hepatis.
131,
132
A commercially available monoclonal antibody specific for B. henselae is also available
and has been used to demonstrate the organism in a case of spontaneous splenic rupture
caused by this bacterium.
133
Fig. 2.14
Bartonella. Immunohistologic demonstration of Bartonella henselae within heart valve
of patient with culture-negative endocarditis. Mouse monoclonal anti-B. henselae antibody.
(Naphthol fast red substrate and hematoxylin counterstain, original magnification
×40)
Other bacterial infections
Other bacterial diseases that can be identified by immunohistochemistry in formalinfixed
tissue including leptospirosis, which is a zoonosis that usually presents as an acute
febrile syndrome but occasionally can have unusual manifestations such as pulmonary
hemorrhage with respiratory failure or abdominal pain.134, 135, 136 Rabbit polyclonal
antibodies have been used in immunohistochemistry to detect leptospiral antigens in
the gallbladder and lungs from patients with unusual presentations (Fig. 2.15
).134, 135, 136, 137
Fig. 2.15
Leptospira. Immunostaining of intact leptospires and granular forms of leptospiral
antigens in kidney of patient who died of pulmonary hemorrhage. (Immunoalkaline phosphatase
with rabbit polyclonal antisera with naphthol fast red substrate and hematoxylin counterstain,
original magnification ×63)
Lyme disease has protean clinical manifestations, and Borrelia burgdoferi is difficult
to culture from tissues and fluids. In addition, cultures are rarely positive before
2–4 weeks of incubation. Borrelia burgdoferi can be identified in tissues by immunostaining
with polyclonal or monoclonal antibodies. Although immunohistochemistry is more specific
than silver impregnation stains, the sensitivity of immunostaining is poor, and the
microorganisms are difficult to detect due to the low numbers present in tissue sections.
138,
139
Immunohistochemistry is useful in identifying Haemophilus influenzae,140, 141, 142
Chlamydia species,143, 144, 145
Legionella pneumophila and L. dumoffii,146, 147, 148
Listeria monocytogenes,149, 150, 151 Salmonella,
152,
153
mycobacteria,154, 155, 156, 157, 158, 159 rickettsial infections other than Rocky
Mountain spotted fever such as boutonneuse fever, typhus fever,
160
rickettsialpox,
161,
162
African tick bite fever,
125
scrub typhus,
163
and spirochetes in patients with syphilis.164, 165, 166
FUNGAL INFECTIONS
The great majority of fungi are readily identified by hematoxylin and eosin staining
alone or in combination with histochemical stains (periodic acid–Schiff [PAS], and
Gomori's methenamine silver [GMS]). However, these stains cannot distinguish morphologically
similar fungi with potential differences in susceptibility to antimycotic drugs. In
addition, fungal elements may appear atypical in tissue sections because of several
factors including steric orientation, age of the fungal lesion, effects of antifungal
chemotherapy, type of infected tissue, and host immune response.
167
Currently, the final identification of fungi relies on culture techniques; however,
culture may take several days or longer to yield a definitive result, and often surgical
pathologists have no access to fresh tissue.
In past years, immunohistochemistry has been used to identify various fungal elements
in paraffin-embedded, formalinfixed tissue.168, 169, 170 Immunohistochemical methods
have the advantage of providing rapid and specific identification of several fungi
and allowing pathologists to be able to identify unusual filamentous hyphal and yeast
infections and accurately distinguish them from confounding artifacts.
169,
172
In addition, immunohistochemistry allows pathologists to correlate microbiological
and histological findings of fungal infections and to distinguish them from harmless
colonization. Immunohistochemistry can also be helpful when more than one fungus is
present; in these cases dual immunostaining techniques can highlight the different
fungal species present in the tissue.
173
An important limitation of immunohistochemistry in the identification of fungi is
the well-known, widespread occurrence of common antigens among pathogenic fungi that
frequently results in cross-reactivity with polyclonal antibodies and even with some
monoclonal antibodies.169, 171, 172, 173, 174 Therefore, assessment of cross-reactivity
using a panel of fungi is a very important step in the evaluation of immunohistochemical
methods.
169,
170
Candida species are often stained weakly with hematoxylin and eosin, and sometimes
the yeast form may be difficult to differentiate from Histoplasma capsulatum, Cryptococcus
neoformans, and even Pneumocystis carinii. Polyclonal and monoclonal antibodies against
Candida genus antigens are sensitive and strongly reactive and do not show cross-reactivity
with other fungi tested.
169,
170,
175,
176
In particular, two monoclonal antibodies against Candida albicans mannoproteins show
high sensitivity and specificity. Monoclonal antibody 3H8 recognizes primarily filamentous
forms of C. albicans, whereas monoclonal antibody 1B12 highlights yeast forms.
176,
177
Identification of Cryptococcus neoformans usually is not a problem when the fungus
produces a mucicarmine-positive capsule. However, infections by capsule-negative strains
are more difficult to diagnose, and the disease can be confused with histoplasmosis,
blastomycosis, or torulopsis. Also, in longstanding infections the yeast often appear
atypical and fragmented. Polyclonal antibodies raised against C. neoformans yeast
cells are sensitive and specific.
169,
170
More recently, monoclonal antibodies have been produced that allow identification
and differentiation of varieties of C. neoformans in formalinfixed tissue. The antibodies
are highly sensitive (97%) and specific (100%) to differentiate C. neoformans var.
neoformans from C. neoformans var. gattii.
178,
179
Sporothrix schenckii may be confused in tissue sections with Blastomyces dermatitidis
and fungal agents of pheohyphomycosis. In addition, yeast cells of S. schenckii may
be sparsely present in tissues. Specific antibodies against yeast cells of S. schenckii
are sensitive but demonstrate cross-reactivity with Candida species; however, after
specific adsorption of the antibody with Candida yeast cells, the cross-reactivity
of the antibodies is eliminated.
169,
170
Invasive aspergillosis is a frequent cause of fungal infection with high morbidity
and mortality rates in immunocompromised patients. The diagnosis is often difficult
and relies heavily on histologic identification of invasive septate hyphae and culture
confirmation. Nevertheless, several filamentous fungi such as Fusarium species, Pseudallescheria
boydii, and Scedosporium species share similar morphology with Aspergillus species
in hematoxylin and eosin-stained tissues. In addition, the yield of cultures in histologically
proven cases is low, ranging from 30% to 50%.
180,
181
Several polyclonal and monoclonal antibodies against Aspergillus antigens have been
tested in formalinfixed tissues with variable sensitivities, and most of them cross-react
with other fungi.
174,
182,
183
More recently, monoclonal antibodies (WF-AF-1, 164G and 611F) against Aspergillus
galactomannan have shown high sensitivity and specificity in identifying A. fumigatus,
A. flavus, and A. niger in formalinfixed tissues without cross-reactivity with other
filamentous fungi.
181,
184
Cysts and trophozoites of Pneumocystis carinii can be detected in bronchoalveolar
lavage specimens using monoclonal antibodies that yield results that are slightly
more sensitive than GMS, Giemsa or Papanicolaou staining for detecting cysts (Fig.
2.16
).
170,
185,
186
Antibodies are most helpful in the diagnosis of P. carinii pneumonia (PCP) when atypical
pathologic features are present such as granulomatous PCP or the presence of hyaline
membranes or in cases of extrapulmonary pneumocystosis.
Fig. 2.16
Human immunodeficiency virus (HIV)-infected immunodeficient patient with Pneumocystis
carinii pneumonia. Cohesive aggregates of cyst forms and trophozoites within alveolar
spaces are demonstrated with a monoclonal antibody against P. carinii in an immunoperoxidase
technique. (DAB with hematoxylin counterstain, ×400)
Penicillium marneffei usually causes a disseminated infection in immunocompromised
patients that clinically resembles histoplasmosis or leishmaniasis.
171,
187
Morphologically, the organisms must be differentiated from H. capsulatum, C. neoformans,
and C. albicans. The monoclonal antibody EBA-1 against the galactomannan of Aspergillus
species cross-reacts with and detects P. marneffei in tissue sections.
182,
188
Immunohistochemistry has also been used to detect Blastomyces, Coccidioides, and Histoplasma.
169,
170,
189
However, the antibodies have significant cross-reactivity with several other fungi.
PROTOZOAL INFECTIONS
Protozoa usually can be identified in tissue sections stained with hematoxylin and
eosin or Giemsa stain; however, because of the small size of the organisms and the
subtle distinguishing features, an unequivocal diagnosis cannot always be made. The
role of immunohistochemistry in the detection of protozoal infections has been limited
to cases in which the morphology of the parasite is distorted by tissue necrosis or
autolysis. In addition, in immunocompromised patients, toxoplasmosis can have an unusual
disseminated presentation with numerous tachyzoites without bradyzoites (Fig. 2.17
).
190,
191
Immunohistochemistry has also been useful in cases with unusual presentation of the
disease.
192
Fig. 2.17
HIV-infected patient with toxoplasmic encephalitis. Immunoperoxidase highlights pseudocysts
and scattered tachyzoites. (DAB with hematoxylin counterstain, ×400).
The diagnosis of leishmaniasis in routine practice usually is not difficult; however,
in certain circumstances the pathologic diagnosis may be more problematic as is the
case in chronic granulomatous leishmaniasis with small numbers of parasites, when
the microorganism presents in unusual locations, or when necrosis distorts the morphologic
appearance of the disease.
193
In these cases, immunohistochemical staining has been a valuable diagnostic tool.193,
194, 195, 196 The highly sensitive and specific monoclonal antibody p19-11 recognizes
different species of Leishmania and allows differentiation from morphologically similar
microorganisms (Toxoplasma, Trypanosoma cruzi, and P. marneffei).
193
Immunohistochemistry has also been used to identify Cryptosporidium,
197
Entamoeba histolytica,
198
Trypanosoma cruzi,199, 200, 201 babesia,
202
and Giardia lamblia
203
in formalinfixed, paraffin-embedded tissue samples.
EMERGING INFECTIOUS DISEASES
In 1992, the Institute of Medicine defined emerging infectious diseases (EID) as caused
by new, previously unidentified microorganisms or those whose incidence in humans
has increased within the past two decades or threatens to increase in the near future.
204
The list of pathogens newly recognized since 1973 is long and continues to increase,
and recognizing emerging infections is a challenge with many new infectious agents
remaining undetected for years before emerging as identified public health problems.
205
EID are global phenomena that require a global response. The Centers for Disease Control
(CDC) has defined the strategy to prevent and detect EID.
205
The anatomic pathology laboratory plays a critical role in the initial and rapid detection
of EID.
206,
207
Immunohistochemistry, besides assisting in the identification of new infectious agents,
has contributed to the understanding of the pathogenesis and epidemiology of EID.
Hantavirus pulmonary syndrome
In 1993, several previously healthy individuals died of rapidly progressive pulmonary
edema, respiratory insufficiency and shock in southwestern United States.
208,
209
Immunohistochemistry was central in the identification of viral antigens of a previously
unknown hantavirus.
210,
211
Immunohistochemical analysis was also important in identifying the occurrence of unrecognized
cases of hantavirus pulmonary syndrome prior to 1993 and in showing the distribution
of viral antigen in endothelial cells of the microcirculation, particularly in the
lung (Fig. 2.18
).
210,
212
Fig. 2.18
Hantavirus antigen-positive endothelial cells of pulmonary microvasculature in lung
of an HPS patient as determined by immunohistochemistry using a mouse monoclonal antibody.
(Immunoalkaline phosphatase with naphthol fast red substrate and hematoxylin counterstain,
original magnification ×100)
West Nile virus encephalitis
West Nile virus (WNV) was originally identified in Africa in 1937, and the first cases
of WNV encephalitis in the US were described in 1999. The clinical picture is variable
and non-specific ranging from subclinical to flaccid paralysis and encephalitis characterized
morphologically by perivascular mononuclear cell inflammatory infiltrates, neuronal
necrosis, edema, and microglial nodules, particularly prominent in the brainstem,
cerebellum, and spinal cord.213, 214, 215, 216, 217 The diagnosis of WNV is usually
established by identification of virus-specific IgM in CSF and/or serum, and demonstration
of viral RNA in serum, CSF, or other tissue.
218
Immunostaining with either monoclonal or polyclonal antibodies has been successfully
employed to diagnose WNV infection in immunocompromised patients who lacked an adequate
antibody response (Fig. 2.19
).
214
Fig. 2.19
West Nile virus. Immunostaining of flaviviral antigens in neurons and neuronal processes
in CNS tissue from an immunosuppressed patient who died of WNV encephalitis. (Flavivirus-hyperimmune
mouse ascitic fluid, naphthol fast red substrate and hematoxylin counterstain, original
magnification ×40)
Enterovirus 71 encephalomyelitis
Enterovirus 71 (EV71) has been associated with hand, foot, and mouth disease, herpangina,
aseptic meningitis, and poliomyelitis-like flaccid paralysis. More recently, EV71
has been associated with unusual cases of fulminant encephalitis, pulmonary edema
and hemorrhage, and heart failure.
219,
220
Severe and extensive encephalomyelitis of the cerebral cortex, brainstem, and spinal
cord has been described. Immunohistochemical staining with monoclonal antibody against
EV71 has played a pivotal role in the linking of EV71 infection to fulminant encephalitis
(Fig. 2.20
). Viral antigen is observed within neurons, neuronal processes, and mononuclear inflammatory
cells.221, 222, 223
Fig. 2.20
Enterovirus 71. Positive staining of EV71 viral antigens in neurons and neuronal processes
of a fatal case of enterovirus encephalitis. (Immunoalkaline phosphatase with naphthol
fast red substrate and hematoxylin counterstain, original magnification ×40)
Nipah virus infection
Nipah virus is a recently described paramyxovirus that causes an acute febrile encephalitic
syndrome with high mortality rates.224, 225, 226 Pathology played a key role in identifying
the causative agent. Histopathologic findings include vasculitis with thrombosis,
microinfarctions, syncytial giant cells, and viral inclusions.
224,
226
Syncytial giant endothelial cells, albeit characteristic of this disease, are seen
only in 25% of cases,
224
and viral inclusions of similar morphology can be seen in other paramyxoviral infections.
Immunostaining provides a useful tool for unequivocal diagnosis of the disease, demonstrating
viral antigen within neurons and endothelial cells of most organs (Fig. 2.21
).
5,
224
Fig. 2.21
Nipah virus. Immunostaining of Nipah virus antigens in neurons and neuronal processes
in CNS of a fatal case of Nipah virus encephalitis. (Naphthol fast red substrate and
hematoxylin counterstain, original magnification ×63)
Ehrlichioses
Bacteria belonging to the genera Ehrlichia and Anaplasma are the agents of human monocytotropic
ehrlichiosis, and human granulocytotropic anaplasmosis, respectively. The acute febrile
illnesses usually present with cytopenias, myalgias, and mild to moderate hepatitis.227,
228, 229, 230
Diagnosis of ehrlichiosis depends upon finding the characteristic monocytic and/or
granulocytic cytoplasmic inclusions (morulae), PCR analysis of blood, and detection
of specific antibodies in blood. However, morulae are rare and often missed on initial
evaluation, hematoxylin and eosin-stained sections often fail to show organisms even
when immunohistochemistry reveals abundant ehrlichial antigen, and antibody titers
may take several weeks to rise to diagnostic levels.
227
Additionally, immunocompromised patients may not develop anti-ehrlichial antibodies
prior to death.
227,
229
In these cases, immunostaining for Ehrlichia or Anaplasma has been demonstrated to
be a sensitive and specific diagnostic method.227, 229, 230, 231
Immunohistochemistry has been a very valuable approach for the identification and
study of several other EID such as Ebola hemorrhagic fever,81, 82, 83 hendra virus
encephalitis,
5,
232,
233
leptospirosis,135, 136, 137 and more recently to identify a new coronavirus associated
with severe acute respiratory syndrome (SARS).
234,
235
SARS was first recognized during a global outbreak of severe pneumonia that first
occurred in late 2002 in Guangdong Province, China, and then erupted in February 2003
with cases in more than two dozen countries in Asia, Europe, North America, and South
America. Early in the investigation the clinical, pathologic, and laboratory studies
focused on previously known agents of respiratory illness. Subsequently, a virus was
isolated from the oropharynx of a SARS patient and identified by ultrastructural characteristics
as belonging to the family Coronaviridae.
234,
235
Various reports have described diffuse alveolar damage as the main histopathologic
findings in SARS patients, and SARS-associated coronavirus (SARS-CoV) has been demonstrated
in human and experimental animal tissues by immunohistochemical (Fig. 2.22
) or in situ hybridization (ISH) assays.236, 237, 238, 239, 240, 241, 242, 243, 244,
245
Fig. 2.22
SARS. Coronavirus antigen-positive pneumocytes and macrophages in lung of a SARS case.
(Immunoalkaline phosphatase with naphthol fast red substrate and hematoxylin counterstain,
original magnification ×63)
PATHOLOGISTS, IMMUNOHISTOCHEMISTRY, AND BIOTERRORISM
Currently, there is increasing concern about the use of infectious agents as potential
biological weapons. Biological warfare agents vary from rare, exotic viruses to common
bacterial agents, and the intentional use of biologic agents to cause disease can
simulate naturally occurring outbreaks or may have unusual characteristics.
246
The CDC has issued the recommendations for a complete public health response to a
biological attack.247, 248, 249 Two important components of this response plan include
the rapid diagnosis and characterization of biological agents. Pathologists using
newer diagnostic techniques such as immunohistochemistry, in situ hybridization, and
PCR will have a direct impact on the rapid detection and control of emerging infectious
diseases from natural or intentional causes. Immunohistochemistry provides a simple,
safe, sensitive, and specific method for the rapid detection, either at the time of
investigation or retrospectively, of biological threats, facilitating the rapid implementation
of effective public health responses.
Anthrax
Immunohistochemical staining of Bacillus anthracis with monoclonal antibodies against
cell wall and capsule antigens has been successfully used in the identification of
bioterrorism-related anthrax cases, being an important step in the early diagnosis
and treatment of these cases.5, 250, 251, 252, 253, 254 Gram's staining and culture
isolation of B. anthracis are the usual methods to diagnose anthrax; nevertheless,
previous antibiotic treatment affects culture yield and Gram's staining identification
of the bacteria.
252
Immunohistochemistry has demonstrated high sensitivity and specificity for the detection
of B. anthracis in skin biopsies, pleural biopsies, transbronchial biopsies, and pleural
fluids (Fig. 2.23
).251, 252, 253
Fig. 2.23
Anthrax. (A) Photomicrograph of pleural effusion cell block showing bacillary fragments
and granular antigen-staining using the B. anthracis capsule antibody. (Immunoalkaline
phosphatase with naphthol fast red substrate and hematoxylin counterstain, original
magnification ×63) (B) Skin biopsy from a patient with cutaneous anthrax showing abundant
granular antigen-staining and bacillary fragments using B. anthracis cell wall antibody.
(Immunoalkaline phosphatase with naphthol fast red substrate and hematoxylin counterstain,
original magnification ×40) (C) Photomicrograph of mediastinal lymph node showing
abundant granular antigen-staining and bacillary fragments using B. anthracis cell
wall antibody. (Immunoalkaline phosphatase with naphthol fast red substrate and hematoxylin
counterstain, original magnification ×63)
In addition, immunostaining has been very useful for determining the route of entry
of the bacteria and identification of the mode of spread of the disease.
252,
255
Tularemia
Immunohistochemical staining is also valuable in the rapid identification of Francisella
tularensis in formalinfixed tissue sections. Tularemia can have a variable clinical
and pathologic presentation that can simulate other infectious diseases such as anthrax,
plague, cat-scratch disease, or lymphogranuloma venereum. Moreover, the microorganisms
are difficult to demonstrate in tissue sections even with Gram's stain or silver staining
methods. A mouse monoclonal antibody against the lipopolysaccharide of F. tularensis
has been used to demonstrate intact bacteria and granular bacterial antigen in the
lungs, spleen, lymph nodes, and liver with high sensitivity and specificity (Fig.
2.24
).
256,
257
Fig. 2.24
Tularemia. Immunohistochemistry of lymph node showing a stellate abscess with F. tularensis
antigen bearing macrophages in the central necrotic area using a mouse monoclonal
antibody against the lipopolysaccharide of F. tularensis. (Immunoalkaline phosphatase
with naphthol fast red substrate and hematoxylin counterstain, original magnification
×10)
Plague
A mouse monoclonal antibody directed against the fraction 1 antigen of Yersinia pestis
has been used to detect intracellular and extracellular bacteria in dermal blood vessels,
lungs, lymph nodes, spleen, and liver (Fig. 2.25
).258, 259, 260, 261, 262 This technique is potentially useful for the rapid diagnosis
of plague in formalinfixed skin biopsies. In addition, immunohistochemistry may allow
distinction of primary and secondary pneumonic plague by
identifying Y. pestis in different lung locations (i.e., alveolar versus interstitial).
258
Fig. 2.25
Plague. Immunohistochemical stain of a lung demonstrating abundant bacterial and granular
antigen staining in the alveolar spaces using a mouse monoclonal antibody against
F1 of Y. pestis. (Immunoalkaline phosphatase with naphthol fast red substrate and
hematoxylin counterstain, original magnification ×20)
Immunohistochemical methods using polyclonal or monoclonal antibodies have been applied
to the identification of several other potential biological terrorism agents, including
antibodies to the causative agents of brucellosis,
5
Q fever,
5,
125,
263,
264
viral encephalitides (Eastern equine encephalitis) (Fig. 2.26
),5, 109, 110, 111 rickettsioses (typhus and Rocky Mountain spotted fever),125, 126,
127, 128, 160 and viral hemorrhagic fevers (Ebola, Marburg).5, 77, 78, 79, 80, 81,
82, 83
Fig. 2.26
Immunostaining of viral antigens in neurons and neuronal processes in CNS using a
mouse anti-EEE antibody. (Immunoalkaline phosphatase with naphthol fast red substrate
and hematoxylin counterstain, original magnification ×10)