In the 64 years since preventive therapy for latent tuberculosis (TB) was first pioneered
by Comstock and colleagues in Alaska, impressive treatment shortening has been achieved
with two simple drug classes: isoniazid and rifamycins. In the last decade, the duration
of therapy for latent TB infection (LTBI) has progressively decreased, going from
9 months to 3 to 4 months, and perhaps to 1 month. To guide these advances and design
phase 3 prevention trials, investigators have designed and tested regimens of antimicrobial
treatment in the chronic low-dose mouse model. Inconveniently, mice do not develop
latent TB. Thus, to estimate drug/regimen efficacy, researchers have assessed rates
of bacterial burden decline in mice as a surrogate for LTBI efficacy, coupled to a
leap of faith.
In this issue of the Journal, Foreman and colleagues (pp. 469–477) now present a dramatic
animal study suggesting that the novel 3-month 12-dose isoniazid–rifapentine (3HP)
regimen for preventive treatment of LTBI has sterilizing capacity (1). For the TB
world, these are impressive findings; validation of these findings in other, related
settings would provide important support for the large-scale use of such interventions.
Scale-up of this intervention is already underway in both low-burden (2) and high-burden/high-HIV
settings (3), so the short-term validation may be soon achieved.
There are important strengths to the study, and there are important questions remaining.
The study was performed in a well-documented nonhuman primate model (the rhesus macaque)
in which the pathophysiology of tuberculosis appears to recapitulate the human analog
well. In the particular model employed here, low-dose infection with Mycobacterium
tuberculosis (MTB) leads to a state of chronic infection; in their study, rapid progression
to active disease occurred in 2 of 16 animals, whereas the remaining 14 remained stable
with minimal or no signs of active disease. In a classic approach, 3 months after
infection, half the remaining infected animals were treated with the 3HP regimen administered
in feed, and half received no treatment. At 7 months after infection, all animals
received an infectious intravenous dose of simian immunodeficiency virus, leading
to a well-recognized state of immune impairment in this host. High simian immunodeficiency
virus viral loads were documented. After a 3-month period of observation, all surviving
animals were killed, and multiple tissues (including lung, bronchial lymph nodes,
liver, spleen, and kidney) from all 14 were cultured on solid media. Cultures were
positive in all untreated animals, whereas only one culture from one of seven treated
animals yielded a single colony on one plate. These results are indeed dramatic. On
their surface, they may indicate that short-course rifamycin-based regimens for LTBI
are highly sterilizing; in that case, the wide application of such regimens could
presage a major step forward in TB prevention and control.
We have two sets of questions that help to place these results in perspective, and
that temper these hopes with scientific caution. Our first questions concern the extent
to which this model replicates the human response to MTB. The authors cataloged clinical
parameters including tuberculin skin test conversion and chest X-ray scoring, as well
as body weight and temperature, throughout the experiment. And as one would expect
to see in humans, tuberculin skin test conversion occurred in the majority of animals
by 30 days and in all of the animals by 70 days, and serial chest X-rays were essentially
negative throughout until the reactivation phase. Importantly, the postmortem pathology
observed in the nonhuman primates closely paralleled that of humans.
Our second set of (related) questions concerns exactly what biological state is being
modeled. In recent years, there has been increasing evidence that TB infection exists
in humans not as a binary condition, of latent TB infection versus active TB disease
(4), but rather as a spectrum that extends from latent, quiescent infection, perhaps
residing only within selected sites or cell lines, to incipient and subclinical disease
(5), to active TB disease (6). More specifically, one must ask whether the condition
being treated in these rhesus macaques is representative of truly persistent/latent
infection, whether it is more similar to subclinical disease, or whether it is parallel
to the human state in the first year after infectious exposure. The importance of
this question is readily apparent: If true persistence is not resolved, then the long-term
risk of reactivation has not been eliminated, and the efficacy of the intervention,
although still significant, may be less than what we hope. In this regard, we wonder
whether the results of this study indicate actual sterilization of MTB in the host
animals. To address some of this concern, the authors performed BAL before giving
3HP and documented culture-negativity. Nevertheless, our concerns arise from two considerations:
first, numerous authors have presented evidence that persistent MTB bacilli are notably
challenging to cultivate, and that large numbers of bacilli that are nonculturable
with routine solid media can be recovered if specimens are cultured in liquid media
after adding culture filtrates or resuscitation promotion factors (7, 8); and second,
the sensitivity of culture using solid media has low but important limits. It is to
the authors’ credit that multiple tissues were examined for acid-fast bacilli and
that multiple tissues were cultured; recent work has suggested that lymph nodes may
represent sites of likely persistence (9).
The careful work by Foreman and colleagues represents an important addition to our
understanding of latent TB infection; evaluation of the current scale-up efforts will
contribute importantly to our understanding of the limits of the model.