Human T-cell leukemia viruses type 1 and 2 (HTLV-1 and HTLV-2) share a common genetic
organization, expression strategy and ability to infect and immortalize T-cells in
vitro; however, HTLV-1 and HTLV-2 are strikingly different in terms of clinical impact.
HTLV-1 is recognized as the aetiological agent of adult T-cell leukemia/lymphoma (ATLL),
and HTLV-associated myeolopathy/tropical spastic paraparesis (HAM/TSP), in contrast,
HTLV-2 does not cause hematologic disorders and is only sporadically associated with
cases of subacute myelopathy. HTLV-1 and HTLV-2 also exhibit distinct cellular tropisms
in vivo: HTLV-1 is mainly found in CD4+T lymphocytes, whereas CD8+T-cells are the
preferred target for HTLV-2.
The articles contributed in this Research Topic are covering all the different aspects
that characterize HTLV-1 and HTLV-2, by highlighting differences in their biology
that might provide clues to their distinct pathogenic properties.
Emphasis was placed on the comparison of mRNA expression (Cavallari et al., 2013),
the genetic organization and expression patterns (Ciminale et al., 2014), the structural
and functional properties of HTLV-1 and HTLV-2 Tax proteins (Forlani et al., 2013;
Ren and Cheng, 2013; Romanelli et al., 2013; Shirinian et al., 2013), the role of
accessory proteins and antisense proteins in viral pathogenesis (Anupam et al., 2013;
Barbeau et al., 2013), and the mechanisms of HIV-1/HTLV-1 and HIV-1/HTLV-2 co-infections
(Pilotti et al., 2013).
The fine tuning of expression of HTLV-1 and HTLV-2 (Cavallari et al., 2013) was focused
on the X region that codes for the regulatory and accessory proteins in partially
overlapping ORFs. Expression of such compact genomes is accomplished by a combination
of ribosomal frameshifting, alternative splicing and polycistronic translation as
well as production of negative-strand transcripts that code for the HBZ and APH-2
proteins. Recent studies of the temporal sequence of HTLV-1 gene expression in PBMCs
isolated from infected patients revealed a “two-phase” kinetics in which tax/rex mRNA
expression precedes that of other viral transcripts. A similar analysis of HTLV-2
mRNA expression indicated a comparable expression pattern, although HTLV-2 is characterized
by a more abundant expression of gene products that may favor viral latency (i.e.,
p28 and tRex).
Given the relevance of Tax in the initial steps of T-cell transformation, their key
role as the main viral oncoproteins was discussed in four different articles, each
one focusing on a specific aspect of Tax function. Romanelli et al. (2013), highlighted
distinctive properties of Tax proteins with emphasis on the activation of the NF-κB
pathways and interactions with host factors that participate in signal trasduction.
Tax-1 and Tax-2, though sharing many common properties, significantly differ for the
presence of a PDZ motif, which is missing in Tax-2. This motif mediates the interaction
of Tax with host factors regulating the cell cycle; in addition Tax-2 is unable to
activate the non-canonical NF-κB which is attributed only to Tax-1. Shirinian et al.
(2013), specifically addressed the subcellular localization and trafficking of Tax-1
and Tax-2, and their effects on cellular regulatory proteins. A special attention
was given to Tax-1/Tax-2 post-translational modifications, as well to NF-κB activation
and protein-protein interactions involved in oncogenecity both in vivo and in vitro.
Ren and Cheng (2013), discussed their recent observations and views on the differential
transforming activity of Tax-1 and Tax-2 in human T cells. Both proteins are unable
to immortalize CD8 cells but Tax-2 is more efficient than Tax-1 in immortalizing CD4
cells. Forlani et al. (2013), discussed the pivotal role of the class II transactivator
(CIITA) in triggering the adaptive immune response against pathogens. CIITA acts as
an endogenous restriction factor against HTLV-1 and HTLV-2 by targeting their viral
transactivators Tax-1 and Tax-2. The authors reviewed their findings on CIITA-mediated
inhibition of viral replication and discussed similarities and differences in the
molecular mechanisms by which CIITA specifically counteracts the function of Tax proteins.
The function of the Rex protein was presented by Ciminale et al. (2014). Although
Rex-1 and Rex-2 share a similar domain structure, the truncated forms of Rex-2 are
capable of inhibiting Rex function, while their HTLV-1 homolog p21 Rex might lack
this activity.
The role in viral replication and viral pathogenesis of accessory proteins HTLV-1
p30 and HTLV-2 p28 were highlighted by Anupam et al. (2013). Both p30 and p28 regulate
viral gene expression at the post-transcriptional level whereas p30 can also function
at the transcriptional level. Since p30 and p28 have distinct interactome profiles
even though they both interact with similar proteins, they must have divergent functions
during the lifecycle of the viruses.
The functions of other accessory proteins have been presented by Ciminale et al. (2014).
HTLV-1 p13 and p8 are not expressed by HTLV-2 which codes for p11, whose function
is still unclear and does not seem to have a homolog in HTLV-1.
The possible role played by antisense proteins HTLV-1 HBZ and HTLV-2 APH-2 in the
establishment of pathologies induced by viral infection has been carefully considered
by Barbeau et al. (2013). Unlike APH-2, HBZ possesses specific domains which mediate
its capacity to enhance its own expression, deregulate several pathways and induce
T cell proliferation, providing substantial evidence toward its implication in viral
persistence and ATLL cell survival.
Finally the problem of HIV/HTLV co-infection has been discussed by Pilotti et al.
(2013). During co-infection with HIV-1, HTLV-2 modulates the cellular microenvironment
favoring its own propagation and inhibiting HIV-1 progression. Possible differences
between HTLV-1 and HTLV-2 on innate immune mechanisms induction and a particularly
impact on NK cells are becoming evident.
In conclusion, in this Research Topic, HTLV-1 and HTLV-2 have been thoroughly compared
and major differences outlined and discussed. Of particular interest is that HTLV-2
infection, contrary to HTLV-1, is characterized by an oligoclonal distribution in
asymptomatic hosts; this indicates that the etiology of malignant transformation by
HTLV-1 cannot be uniquely attributed to this phenomenon and further investigation
is needed to understand why HTLV-2 is defective in promoting leukemogenesis.
Conflict of interest statement
The author declares that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.