Addendum: Immune clearance of highly pathogenic SIV infection.

This corrects the article DOI: 10.1038/nature12519.

Immune clearance of highly pathogenic SIV infection.

Established infections with the human and simian immunodeficiency viruses (HIV and SIV, respectively) are thought to be permanent with even the most effective immune responses and antiretroviral therapies only able to control, but not clear, these infections. Whether the residual virus that maintains these infections is vulnerable to clearance is a question of central importance to the future management of millions of HIV-infected individuals. We recently reported that approximately 50% of rhesus macaques (RM; Macaca mulatta) vaccinated with SIV protein-expressing rhesus cytomegalovirus (RhCMV/SIV) vectors manifest durable, aviraemic control of infection with the highly pathogenic strain SIVmac239 (ref. 5). Here we show that regardless of the route of challenge, RhCMV/SIV vector-elicited immune responses control SIVmac239 after demonstrable lymphatic and haematogenous viral dissemination, and that replication-competent SIV persists in several sites for weeks to months. Over time, however, protected RM lost signs of SIV infection, showing a consistent lack of measurable plasma- or tissue-associated virus using ultrasensitive assays, and a loss of T-cell reactivity to SIV determinants not in the vaccine. Extensive ultrasensitive quantitative PCR and quantitative PCR with reverse transcription analyses of tissues from RhCMV/SIV vector-protected RM necropsied 69-172 weeks after challenge did not detect SIV RNA or DNA sequences above background levels, and replication-competent SIV was not detected in these RM by extensive co-culture analysis of tissues or by adoptive transfer of 60 million haematolymphoid cells to naive RM. These data provide compelling evidence for progressive clearance of a pathogenic lentiviral infection, and suggest that some lentiviral reservoirs may be susceptible to the continuous effector memory T-cell-mediated immune surveillance elicited and maintained by cytomegalovirus vectors.

Neisseria infection of rhesus macaques as a model to study colonization, transmission, persistence, and horizontal gene transfer.

The strict tropism of many pathogens for man hampers the development of animal models that recapitulate important microbe-host interactions. We developed a rhesus macaque model for studying Neisseria-host interactions using Neisseria species indigenous to the animal. We report that Neisseria are common inhabitants of the rhesus macaque. Neisseria isolated from the rhesus macaque recolonize animals after laboratory passage, persist in the animals for at least 72 d, and are transmitted between animals. Neisseria are naturally competent and acquire genetic markers from each other in vivo, in the absence of selection, within 44 d after colonization. Neisseria macacae encodes orthologs of known or presumed virulence factors of human-adapted Neisseria, as well as current or candidate vaccine antigens. We conclude that the rhesus macaque model will allow studies of the molecular mechanisms of Neisseria colonization, transmission, persistence, and horizontal gene transfer. The model can potentially be developed further for preclinical testing of vaccine candidates.

Progressive CD4+ central memory T cell decline results in CD4+ effector memory insufficiency and overt disease in chronic SIV infection.

Primary simian immunodeficiency virus (SIV) infections of rhesus macaques result in the dramatic depletion of CD4(+) CCR5(+) effector-memory T (T(EM)) cells from extra-lymphoid effector sites, but in most infections, an increased rate of CD4(+) memory T cell proliferation appears to prevent collapse of effector site CD4(+) T(EM) cell populations and acute-phase AIDS. Eventually, persistent SIV replication results in chronic-phase AIDS, but the responsible mechanisms remain controversial. Here, we demonstrate that in the chronic phase of progressive SIV infection, effector site CD4(+) T(EM) cell populations manifest a slow, continuous decline, and that the degree of this depletion remains a highly significant correlate of late-onset AIDS. We further show that due to persistent immune activation, effector site CD4(+) T(EM) cells are predominantly short-lived, and that their homeostasis is strikingly dependent on the production of new CD4(+) T(EM) cells from central-memory T (T(CM)) cell precursors. The instability of effector site CD4(+) T(EM) cell populations over time was not explained by increasing destruction of these cells, but rather was attributable to progressive reduction in their production, secondary to decreasing numbers of CCR5(-) CD4(+) T(CM) cells. These data suggest that although CD4(+) T(EM) cell depletion is a proximate mechanism of immunodeficiency, the tempo of this depletion and the timing of disease onset are largely determined by destruction, failing production, and gradual decline of CD4(+) T(CM) cells.

Prevention of tuberculosis in rhesus macaques by a cytomegalovirus-based vaccine.

Despite widespread use of the bacille Calmette-Guérin (BCG) vaccine, tuberculosis (TB) remains a leading cause of global mortality from a single infectious agent (Mycobacterium tuberculosis or Mtb). Here, over two independent Mtb challenge studies, we demonstrate that subcutaneous vaccination of rhesus macaques (RMs) with rhesus cytomegalovirus vectors encoding Mtb antigen inserts (hereafter referred to as RhCMV/TB)-which elicit and maintain highly effector-differentiated, circulating and tissue-resident Mtb-specific CD4+ and CD8+ memory T cell responses-can reduce the overall (pulmonary and extrapulmonary) extent of Mtb infection and disease by 68%, as compared to that in unvaccinated controls, after intrabronchial challenge with the Erdman strain of Mtb at ∼1 year after the first vaccination. Fourteen of 34 RhCMV/TB-vaccinated RMs (41%) across both studies showed no TB disease by computed tomography scans or at necropsy after challenge (as compared to 0 of 17 unvaccinated controls), and ten of these RMs were Mtb-culture-negative for all tissues, an exceptional long-term vaccine effect in the RM challenge model with the Erdman strain of Mtb. These results suggest that complete vaccine-mediated immune control of highly pathogenic Mtb is possible if immune effector responses can intercept Mtb infection at its earliest stages.

Technical advance: liposomal alendronate depletes monocytes and macrophages in the nonhuman primate model of human disease.

Nonhuman primates are critical animal models for the study of human disorders and disease and offer a platform to assess the role of immune cells in pathogenesis via depletion of specific cellular subsets. However, this model is currently hindered by the lack of reagents that safely and specifically ablate myeloid cells of the monocyte/macrophage Lin. Given the central importance of macrophages in homeostasis and host immunity, development of a macrophage-depletion technique in nonhuman primates would open new avenues of research. Here, using LA at i.v. doses as low as 0.1 mg/kg, we show a >50% transient depletion of circulating monocytes and tissue-resident macrophages in RMs by an 11-color flow cytometric analysis. Diminution of monocytes was followed rapidly by emigration of monocytes from the bone marrow, leading to a rebound of monocytes to baseline levels. Importantly, LA was well-tolerated, as no adverse effects or changes in gross organ function were observed during depletion. These results advance the ex vivo study of myeloid cells by flow cytometry and pave the way for in vivo studies of monocyte/macrophage biology in nonhuman primate models of human disease.

Naive T cells are dispensable for memory CD4+ T cell homeostasis in progressive simian immunodeficiency virus infection.

The development of AIDS in chronic HIV/simian immunodeficiency virus (SIV) infection has been closely linked to progressive failure of CD4(+) memory T cell (T(M)) homeostasis. CD4(+) naive T cells (T(N)) also decline in these infections, but their contribution to disease progression is less clear. We assessed the role of CD4(+) T(N) in SIV pathogenesis using rhesus macaques (RMs) selectively and permanently depleted of CD4(+) T(N) before SIV infection. CD4(+) T(N)-depleted and CD4(+) T(N)-repleted RMs were created by subjecting juvenile RMs to thymectomy versus sham surgery, respectively, followed by total CD4(+) T cell depletion and recovery from this depletion. Although thymectomized and sham-treated RMs manifested comparable CD4(+) T(M) recovery, only sham-treated RMs reconstituted CD4(+) T(N). CD4(+) T(N)-depleted RMs responded to SIVmac239 infection with markedly attenuated SIV-specific CD4(+) T cell responses, delayed SIVenv-specific Ab responses, and reduced SIV-specific CD8(+) T cell responses. However, CD4(+) T(N)-depleted and -repleted groups showed similar levels of SIV replication. Moreover, CD4(+) T(N) deficiency had no significant effect on CD4(+) T(M) homeostasis (either on or off anti-retroviral therapy) or disease progression. These data demonstrate that the CD4(+) T(N) compartment is dispensable for CD4(+) T(M) homeostasis in progressive SIV infection, and they confirm that CD4(+) T(M) comprise a homeostatically independent compartment that is intrinsically capable of self-renewal.

Profound CD4+/CCR5+ T cell expansion is induced by CD8+ lymphocyte depletion but does not account for accelerated SIV pathogenesis.

Depletion of CD8(+) lymphocytes during acute simian immunodeficiency virus (SIV) infection of rhesus macaques (RMs) results in irreversible prolongation of peak-level viral replication and rapid disease progression, consistent with a major role for CD8(+) lymphocytes in determining postacute-phase viral replication set points. However, we report that CD8(+) lymphocyte depletion is also associated with a dramatic induction of proliferation among CD4(+) effector memory T (T(EM)) cells and, to a lesser extent, transitional memory T (T(TrM)) cells, raising the question of whether an increased availability of optimal (activated/proliferating), CD4(+)/CCR5(+) SIV "target" cells contributes to this accelerated pathogenesis. In keeping with this, depletion of CD8(+) lymphocytes in SIV(-) RMs led to a sustained increase in the number of potential CD4(+) SIV targets, whereas such depletion in acute SIV infection led to increased target cell consumption. However, we found that the excess CD4(+) T(EM) cell proliferation of CD8(+) lymphocyte-depleted, acutely SIV-infected RMs was completely inhibited by interleukin (IL)-15 neutralization, and that this inhibition did not abrogate the rapidly progressive infection in these RMs. Moreover, although administration of IL-15 during acute infection induced robust CD4(+) T(EM) and T(TrM) cell proliferation, it did not recapitulate the viral dynamics of CD8(+) lymphocyte depletion. These data suggest that CD8(+) lymphocyte function has a larger impact on the outcome of acute SIV infection than the number and/or activation status of target cells available for infection and viral production.