Understanding the basis of CD4(+) T-cell depletion in macaques infected by a simian-human immunodeficiency virus

The efficacy of candidate AIDS vaccines to mediate protection against viral infection and pathogenesis is evaluated, at a preclinical stage, in animal models. One model that is favored because the infecting virus is closely related to HIV-1 and because of the rapidity of pathogenic outcomes is the infection of Old World monkeys by simian-human immunodeficiency virus (SHIV) chimerae. We investigated the basis for the depletion of CD4(+) T lymphocytes in a SHIV-macaque model. Molecularly cloned SHIVs, SHIV-89.6 and SHIV-KB9, differ in the ability to cause CD4(+) T-cell loss at a given level of virus replication in monkeys. The envelope glycoproteins of the pathogenic SHIV-KB9 mediate membrane-fusion in cultured T lymphocytes more efficiently than the envelope glycoproteins of the non-pathogenic SHIV-89.6. The minimal envelope glycoprotein region that specifies this increase in membrane-fusing capacity was sufficient to convert SHIV-89.6 into a virus that causes profound CD4(+) T-cell depletion in monkeys. Conversely, two single amino acid changes that decrease the membrane-fusing ability of the SHIV-KB9 envelope glycoproteins also attenuated the CD4(+) T-cell destruction that accompanied a given level of virus replication in SHIV-infected monkeys. Thus, the ability of the HIV-1 envelope glycoproteins to fuse membranes, which has been implicated in the induction of viral cytopathic effects in vitro, contributes to the capacity of the pathogenic SHIV to deplete CD4(+) T lymphocytes in vivo.     

Dramatic increase in naive T cell turnover is linked to loss of naive T cells from old primates

The loss of naïve T cells is a hallmark of immune aging. Although thymic involution is a primary driver of this naïve T cell loss, less is known about the contribution of other mechanisms to the depletion of naïve T cells in aging primates. We examined the role of homeostatic cycling and proliferative expansion in different T cell subsets of aging rhesus macaques (RM). BrdU incorporation and the expression of the G₁-M marker Ki-67 were elevated in peripheral naïve CD4 and even more markedly in the naïve CD8 T cells of old, but not young adult, RM. Proliferating naïve cells did not accumulate in old animals. Rather, the relative size of the naïve CD8 T cell compartment correlated inversely to its proliferation rate. Likewise, T cell receptor diversity decreased in individuals with elevated naïve CD8 T cell proliferation. This apparent contradiction was explained by a significant increase in turnover concomitant with the naïve pool loss. The turnover increased exponentially when the naïve CD8 T cell pool decreased below 4% of total blood CD8 cells. These results link the shrinking naïve T cell pool with a dramatic increase in homeostatic turnover, which has the potential to exacerbate the progressive exhaustion of the naïve pool and constrict the T cell repertoire. Thus, homeostatic T cell proliferation exhibits temporal antagonistic pleiotropy, being beneficial to T cell maintenance in adulthood but detrimental to the long-term T cell maintenance in aging individuals.     

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.     

Rapid CD4+ T-lymphocyte depletion in rhesus monkeys infected with a simian-human immunodeficiency virus expressing the envelope glycoproteins of a primary dual-tropic Ethiopian Clade C HIV type 1 isolate

Simian-human immunodeficiency virus (SHIV) chimerae with the envelope glycoproteins of X4 or R5/X4 HIV-1 isolates from clade B can cause rapid and severe CD4(+) T cell depletion and AIDS-like illness in infected monkeys. We created a SHIV (SHIV-MCGP1.3) expressing the envelope glycoproteins of a primary R5/X4, clade C HIV-1 isolate. Infection of a rhesus monkey with SHIV-MCGP1.3 resulted in a low level of viremia and no significant alteration in CD4(+) T-lymphocyte counts. However, serial intravenous passage of the virus resulted in the emergence of SHIV-MCGP1.3 variants that replicated efficiently and caused profound CD4(+) T cell depletion during the acute phase of infection. The CD4(+) T cell counts in the infected monkeys gradually returned to normal, and the animals remained healthy. The ability to cause rapid and profound loss of CD4(+) T lymphocytes in vivo is a property shared by passaged, CXCR4-using SHIVs, irrespective of the clade of origin of the HIV-1 envelope glycoproteins.     

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.     

Neutralization sensitivity of a simian-human immunodeficiency virus (SHIV-HXBc2P 3.2N) isolated from an infected rhesus macaque with neurological disease

Simian-human immunodeficiency virus (SHIV) chimerae, after in vivo passage in monkeys, can induce acquired immunodeficiency syndrome (AIDS)-like illness and death. A monkey infected with the molecularly cloned, pathogenic SHIV-HXBc2P 3.2 exhibited multifocal granulomatous pneumonia as well as progressive neurological impairment characterized by tremors and pelvic limb weakness. SHIV-HXBc2P 3.2N was isolated from brain tissue explants and characterized. Viruses with the envelope glycoproteins of SHIV-HXBc2P 3.2N exhibited increased sensitivity to soluble CD4 and several neutralizing antibodies compared with viruses with the parental SHIV-HXBc2P 3.2 envelope glycoproteins. By contrast, viruses with SHIV-HXBc2P 3.2 and SHIV-HXBc2P 3.2N envelope glycoproteins were neutralized equivalently by 2G12 and 2F5 antibodies, which are rarely elicited in HIV-1-infected humans. A constellation of changes involving both gp120 and gp41 envelope glycoproteins was responsible for the difference in susceptibility to neutralization by most antibodies. Surprisingly, the gain of an N-linked glycosylation site in the gp41 ectodomain contributed greatly to neutralization sensitivity. Thus, the environment of the central nervous system, particularly in the context of immunodeficiency, allows the evolution of immunodeficiency viruses with greater susceptibility to neutralization by antibodies.     

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.In the initial weeks of HIV infection of humans and pathogenic simian immunodeficiency virus (SIV) infection of Asian macaques, viral replication peaks, then declines to a quasiequilibrated set point of ongoing viral production and clearance, the level of which plays a major role in determining the subsequent tempo of disease progression (Mellors et al., 1996; Staprans et al., 1999). Outcomes range from an inability to substantially restrain viral replication from peak levels, leading to early immunological collapse and rapid progression to AIDS, to control of viral replication to undetectable levels and long-term nonprogression (Farzadegan et al., 1996; Picker et al., 2004; Deeks and Walker, 2007; Goulder and Watkins, 2008). However, the vast majority of infections manifest viral replication set points and progression rates between these two extremes (Munoz et al., 1989; Okoye et al., 2007). The mechanisms responsible for these different outcomes have not been precisely defined, although differences in adaptive immunity, innate immunity, and CD4⁺, CCR5⁺ target cell availability, susceptibility to infection, productivity (viral yield per infected cell), and dynamics have all been implicated (Goldstein et al., 2000; Seman et al., 2000; Zhang et al., 2004; Alter et al., 2007; Goulder and Watkins, 2008; Lehner et al., 2008; Mahalanabis et al., 2009).The HIV/SIV-specific CD8⁺ T cell response has been widely accepted as a major, if not dominant, contributor to this heterogeneity of outcomes based on the observations that (a) the appearance of these responses is temporally coordinated with the postpeak fall in viral replication (Koup et al., 1994), (b) vaccines that elicit strong CD8⁺ T cell responses can lower viral replication set points compared with unvaccinated controls (Wilson et al., 2006; Liu et al., 2009), (c) particular class 1 MHC alleles and their associated CD8⁺ T cell responses are strongly associated with postpeak control of viremia (Goulder and Watkins, 2008), (d) viral mutations facilitating escape from CD8⁺ T cell recognition can be associated with either loss of virologic control or a fitness cost that handicaps replication of escaped virus (Barouch et al., 2002; Goulder and Watkins, 2008), and (e) treatment of rhesus macaques (RMs) with depleting anti-CD8⁺ mAbs at the outset of SIV infection, transiently depleting CD8⁺ lymphocytes from blood and secondary lymphoid tissues, typically results in unrestrained viral replication and rapid disease progression (Matano et al., 1998; Schmitz et al., 1999; Kim et al., 2008; Veazey et al., 2008). On the other hand, there is considerable circumstantial evidence suggesting that the availability, susceptibility to infection, and cumulative per cell virus production of HIV/SIV target cells may also play a major role in determining acute-phase viral dynamics and subsequent viral load set points. In early acute SIV infection, the primary target cells are small, resting CD4⁺, CCR5⁺ T_EM and transitional memory T (T_TrM) cells in tissues; massive infection and destruction of these cells corresponds to the initial peak of viral replication and its subsequent decline (Picker et al., 2004; Li et al., 2005; Mattapallil et al., 2005). With the destruction of resting CD4⁺ target cells and the onset of infection-associated inflammation, the infection shifts to predominant replication in activated, proliferating CD4⁺ T_EM and T_TrM cells (Zhang et al., 2004; Haase, 2005). These observations suggest that in typical SIV infections, plateau-phase viral replication might depend on both the rate of new target cell production and the enhanced per cell virus production of activated target cells. Consistent with this, it has been well documented that both coinfection with other pathogens and other modes of immune activation in acute infection, which increase the number of activated target cells, are associated with increased levels of viral replication and rapid disease progression (Folks et al., 1997; Zhou et al., 1999; Sequar et al., 2002; Garber et al., 2004; Cecchinato et al., 2008).Determination of the interplay between cellular immune effector responses and target cell dynamics in the regulation of acute-phase HIV/SIV replication will be crucial to understand the potential impact of vaccines and other immunomodulators on HIV/SIV pathogenesis. However, experimental dissection of these processes in vivo is complicated by the fact that primary viral targets—CD4⁺ memory T cells—are an intrinsic and essential part of the adaptive immune response and are almost invariably coregulated with CD8⁺ lymphocyte–mediated effector responses. In this paper, we report on the use of CD8⁺ lymphocyte depletion in RMs, which effectively eliminates CD8⁺ effector lymphocyte responses in acute SIV infection (Matano et al., 1998; Schmitz et al., 1999; Kim et al., 2008; Veazey et al., 2008), to determine the importance of target cell expansion and activation on viral dynamics in acute SIV infection. We document that CD8⁺ lymphocyte depletion induces massive selective proliferation of CD4⁺, CCR5⁺ memory T cell targets, leading to profound expansion of this population in uninfected RMs and their increased consumption in acute SIV infection. Furthermore, we identify IL-15 as the major mediator of this proliferative response. Significantly, however, in vivo inhibition of IL-15 by administration of a neutralizing anti–IL-15 mAb during CD8⁺ lymphocyte depletion of acutely SIV-infected RMs blocked induction of CD4⁺ target cell proliferation, but did not reverse either the high viral replication or rapid disease progression in CD8⁺ lymphocyte–depleted, acutely infected animals. Moreover, administration of exogenous IL-15 during acute SIV infection in RMs with an intact CD8⁺ lymphocyte compartment stimulated CD4⁺ target cell proliferation with kinetics similar to CD8⁺ lymphocyte depletion, but did not recapitulate the early viral dynamics of CD8⁺ lymphocyte depletion. Collectively, these data provide compelling evidence that the relative effectiveness of adaptive and/or innate immune function of CD8⁺ lymphocytes plays a considerably more significant role in determining outcome of acute SIV infection than differences in target cell number and/or activation status.