Induction of Transmucosal Protection by Oral Vaccination with an Attenuated Chlamydia.

Chlamydia muridarum has been used to study chlamydial pathogenesis because it induces mice to develop hydrosalpinx, a pathology observed in C. trachomatis-infected women. We identified a C. muridarum mutant that is no longer able to induce hydrosalpinx. In the current study, we evaluated the mutant as an attenuated vaccine. Following an intravaginal immunization with the mutant, mice were protected from hydrosalpinx induced by wild-type C. muridarum. However, the mutant itself productively colonized the mouse genital tract and produced infectious organisms in vaginal swabs. Nevertheless, the mutant failed to produce infectious shedding in the rectal swabs following an oral inoculation. Importantly, mice orally inoculated with the mutant mounted transmucosal immunity against challenge infection of wild-type C. muridarum in the genital tract. The protection was detected as early as day 3 following the genital challenge infection and the orally immunized mice were protected from any significant pathology in the upper genital tract. However, the same orally immunized mice failed to prevent the colonization of wild-type C. muridarum in the gastrointestinal tract. The transmucosal immunity induced by the oral mutant was further validated in the airway. The orally vaccinated mice were protected from both lung infection and systemic toxicity caused by intranasally inoculated wild-type C. muridarum although the same mice still permitted the gastrointestinal colonization by the wild-type C. muridarum. These observations suggest that the mutant C. muridarum may be developed into an intracellular oral vaccine vector (or IntrOv) for selectively inducing transmucosal immunity in extra-gut tissues.

Induction of transmucosal protection by oral vaccination with an attenuated Chlamydia.

Chlamydia muridarum has been used to study chlamydial pathogenesis since it induces mice to develop hydrosalpinx, a pathology observed in C. trachomatis -infected women. We identified a C. muridarum mutant that is no longer able to induce hydrosalpinx. In the current study, we evaluated the mutant as an attenuated vaccine. Following an intravaginal immunization with the mutant, mice were protected from hydrosalpinx induced by wild type C. muridarum . However, the mutant itself productively colonized the mouse genital tract and produced infectious organisms in vaginal swabs. Nevertheless, the mutant failed to produce infectious shedding in the rectal swabs following an oral inoculation. Importantly, mice orally inoculated with the mutant mounted transmucosal immunity against challenge infection of wild type C. muridarum in the genital tract. The protection was detected as early as day 3 following the challenge infection and the immunized mice were protected from any significant pathology in the upper genital tract. However, the same orally immunized mice failed to prevent the colonization of wild type C. muridarum in the gastrointestinal tract. The transmucosal immunity induced by the oral mutant was further validated in the airway. The orally vaccinated mice were protected from both lung infection and systemic toxicity caused by intranasally inoculated wild type C. muridarum although the same mice still permitted the gastrointestinal colonization by the wild type C. muridarum . These observations suggest that the mutant C. muridarum may be developed into an intr acellular o ral v accine vector (or IntrOv) for selectively inducing transmucosal immunity in extra-gut tissues.

Human Macrophages Exhibit GM-CSF Dependent Restriction of Mycobacterium tuberculosis Infection via Regulating Their Self-Survival, Differentiation and Metabolism.

GM-CSF is an important cytokine that regulates the proliferation of monocytes/macrophages and its various functions during health and disease. Although growing evidences support the notion that GM-CSF could play a major role in immunity against tuberculosis (TB) infection, the mechanism of GM-CSF mediated protective effect against TB remains largely unknown. Here in this study we examined the secreted levels of GM-CSF by human macrophages from different donors along with the GM-CSF dependent cellular processes that are critical for control of M. tuberculosis infection. While macrophage of different donors varied in their ability to produce GM-CSF, a significant correlation was observed between secreted levels of GM-CSF, survial of macrophages and intra-macrophage control of Mycobacterium tuberculosis bacilli. GM-CSF levels secreted by macrophages negatively correlated with the intra-macrophage M. tuberculosis burden, survival of infected host macrophages positively correlated with their GM-CSF levels. GM-CSF-dependent prolonged survival of human macrophages also correlated with significantly decreased bacterial burden and increased expression of self-renewal/cell-survival associated genes such as BCL-2 and HSP27. Antibody-mediated depletion of GM-CSF in macrophages resulted in induction of significantly elevated levels of apoptotic/necrotic cell death and a simultaneous decrease in autophagic flux. Additionally, protective macrophages against M. tuberculosis that produced more GM-CSF, induced a stronger granulomatous response and produced significantly increased levels of IL-1ß, IL-12 and IL-10 and decreased levels of TNF-α and IL-6. In parallel, macrophages isolated from the peripheral blood of active TB patients exhibited reduced capacity to control the intracellular growth of M. tuberculosis and produced significantly lower levels of GM-CSF. Remarkably, as compared to healthy controls, macrophages of active TB patients exhibited significantly altered metabolic state correlating with their GM-CSF secretion levels. Altogether, these results suggest that relative levels of GM-CSF produced by human macrophages plays a critical role in preventing cell death and maintaining a protective differentiation and metabolic state of the host cell against M. tuberculosis infection.

Human M1 macrophages express unique innate immune response genes after mycobacterial infection to defend against tuberculosis.

Mycobacterium tuberculosis (Mtb) is responsible for approximately 1.5 million deaths each year. Though 10% of patients develop tuberculosis (TB) after infection, 90% of these infections are latent. Further, mice are nearly uniformly susceptible to Mtb but their M1-polarized macrophages (M1-MΦs) can inhibit Mtb in vitro, suggesting that M1-MΦs may be able to regulate anti-TB immunity. We sought to determine whether human MΦ heterogeneity contributes to TB immunity. Here we show that IFN-γ-programmed M1-MΦs degrade Mtb through increased expression of innate immunity regulatory genes (Inregs). In contrast, IL-4-programmed M2-polarized MΦs (M2-MΦs) are permissive for Mtb proliferation and exhibit reduced Inregs expression. M1-MΦs and M2-MΦs express pro- and anti-inflammatory cytokine-chemokines, respectively, and M1-MΦs show nitric oxide and autophagy-dependent degradation of Mtb, leading to increased antigen presentation to T cells through an ATG-RAB7-cathepsin pathway. Despite Mtb infection, M1-MΦs show increased histone acetylation at the ATG5 promoter and pro-autophagy phenotypes, while increased histone deacetylases lead to decreased autophagy in M2-MΦs. Finally, Mtb-infected neonatal macaques express human Inregs in their lymph nodes and macrophages, suggesting that M1 and M2 phenotypes can mediate immunity to TB in both humans and macaques. We conclude that human MФ subsets show unique patterns of gene expression that enable differential control of TB after infection. These genes could serve as targets for diagnosis and immunotherapy of TB.

High-Resolution Quantitative Mapping of Macaque Cervicovaginal Epithelial Thickness: Implications for Mucosal Vaccine Delivery.

Vaginal mucosal surfaces naturally offer some protection against sexually transmitted infections (STIs) including Human Immunodeficiency Virus-1, however topical preventative medications or vaccine designed to boost local immune responses can further enhance this protection. We previously developed a novel mucosal vaccine strategy using viral vectors integrated into mouse dermal epithelium to induce virus-specific humoral and cellular immune responses at the site of exposure. Since vaccine integration occurs at the site of cell replication (basal layer 100-400 micrometers below the surface), temporal epithelial thinning during vaccine application, confirmed with high resolution imaging, is desirable. In this study, strategies for vaginal mucosal thinning were evaluated noninvasively using optical coherence tomography (OCT) to map reproductive tract epithelial thickness (ET) in macaques to optimize basal layer access in preparation for future effective intravaginal mucosal vaccination studies. Twelve adolescent female rhesus macaques (5-7kg) were randomly assigned to interventions to induce vaginal mucosal thinning, including cytobrush mechanical abrasion, the chemical surfactant spermicide nonoxynol-9 (N9), the hormonal contraceptive depomedroxyprogesterone acetate (DMPA), or no intervention. Macaques were evaluated at baseline and after interventions using colposcopy, vaginal biopsies, and OCT imaging, which allowed for real-time in vivo visualization and measurement of ET of the mid-vagina, fornices, and cervix. P value ≤0.05 was considered significant. Colposcopy findings included pink, rugated tissue with variable degrees of white-tipped, thickened epithelium. Baseline ET of the fornices was thinner than the cervix and vagina (p<0.05), and mensing macaques had thinner ET at all sites (p<0.001). ET was decreased 1 month after DMPA (p<0.05) in all sites, immediately after mechanical abrasion (p<0.05) in the fornix and cervix, and after two doses of 4% N9 (1.25ml) applied over 14 hrs in the fornix only (p<0.001). Histological assessment of biopsied samples confirmed OCT findings. In summary, OCT imaging allowed for real time assessment of macaque vaginal ET. While varying degrees of thinning were observed after the interventions, limitations with each were noted. ET decreased naturally during menses, which may provide an ideal opportunity for accessing the targeted vaginal mucosal basal layers to achieve the optimum epithelial thickness for intravaginal mucosal vaccination.

Cytomegaloviral hypophysitis in a simian immunodeficiency virus-infected rhesus macaque (Macacca mulatta).

Rhesus macaques experimentally infected with Simian Immunodeficiency Virus (SIV) experience immunosuppression and often opportunistic infection. Among the most common opportunistic infections are rhesus cytomegalovirus (RhCMV), a ubiquitous betaherpesvirus that undergoes continuous low-level replication in immunocompetent monkeys. Upon SIV-mediated immunodeficiency, RhCMV reactivates and results in lesions in numerous organ systems including the nervous and reproductive systems. We report the first case of cytomegaloviral hypophysitis in a SIV-immunocompromised rhesus macaque.

Magnitude and Quality of Cytokine and Chemokine Storm during Acute Infection Distinguish Nonprogressive and Progressive Simian Immunodeficiency Virus Infections of Nonhuman Primates.

Acute human immunodeficiency virus (HIV) infection represents a period of intense immune perturbation and activation of the host immune system. Study of the eclipse and viral expansion phases of infection is difficult in humans, but studies in nonprogressive and progressive nonhuman primate (NHP) infection models can provide significant insight into critical events occurring during this time. Cytokines, chemokines, and other soluble immune factors were measured in longitudinal samples from rhesus macaques infected with either SIVmac251 (progressive infection) or SIVmac239Δnef (attenuated/nonprogressive infection) and from African green monkeys infected with SIVsab9315BR (nonpathogenic infection). Levels of acute-phase peak viral replication were highest in SIVmac251 infection but correlated positively with viremia at 3 months postinfection in all three infection models. SIVmac251 infection was associated with stronger corresponding acute-phase cytokine/chemokine responses than the nonprogressive infections. The production of interleukin 15 (IL-15), IL-18, gamma interferon (IFN-γ), granulocyte colony-stimulating factor (G-CSF), monocyte chemoattractant protein 1 (MCP-1), macrophage inflammatory protein 1ß (MIP-1ß), and serum amyloid A protein (SAA) during acute SIVmac251 infection, but not during SIVmac239Δnef or SIVsab9315BR infection, correlated positively with chronic viremia at 3 months postinfection. Acute-phase production of MCP-1 correlated with viremia at 3 months postinfection in both nonprogressive infections. Finally, a positive correlation between the acute-phase area under the curve (AUC) for IL-6 and soluble CD40 ligand (sCD40L) and chronic viremia was observed only for the nonprogressive infection models. While we observed dynamic acute inflammatory immune responses in both progressive and nonprogressive SIV infections, the responses in the nonprogressive infections were not only lower in magnitude but also qualitatively different biomarkers of disease progression. IMPORTANCE: NHP models of HIV infection constitute a powerful tool with which to study viral pathogenesis in order to gain critical information for a better understanding of HIV infection in humans. Here we studied progressive and nonprogressive simian immunodeficiency virus (SIV) infection models in both natural and nonnatural host NHP species. Regardless of the pathogenicity of the virus infection and regardless of the NHP species studied, the magnitude of viremia, as measured by area under the curve, during the first 4 weeks of infection correlated positively with viremia in chronic infection. The magnitude of cytokine and chemokine responses during primary infection also correlated positively with both acute-phase and chronic viremia. However, the pattern and levels of specific cytokines and chemokines produced differed between nonprogressive and progressive SIV infection models. The qualitative differences in the early immune response in pathogenic and nonpathogenic infections identified here may be important determinants of the subsequent disease course.

Epithelial stem cells as mucosal antigen-delivering cells: A novel AIDS vaccine approach.

A key obstacle limiting development of an effective AIDS vaccine is the inability to deliver antigen for a sufficient period of time resulting in weak and transient protection. HIV transmission occurs predominantly across mucosal surfaces; therefore, an ideal vaccine strategy would be to target HIV at mucosal entry sites to prevent infection. Such a novel strategy relies on the activation of mucosal immune response via presentation of viral antigens by the mucosal epithelial cells. The use of a terminally differentiated epithelial cell promoter to drive expression of antigens leading to viral protein production in the upper layers of the epithelium is central to the success of this approach. Our results show that when administered intradermally to mice, a GFP-reporter gene under the transcriptional control of the involucrin promoter is expressed in the upper layers of the epidermis and, although transduced cells were very low in number, high and sustained anti-GFP antibody production is observed in vivo. A subsequent experiment investigates the effectiveness of GFP-tagged replication-competent SIVdeltaNef and GFP-tagged replication-deficient SIVdeltaVifdeltaNef constructs under the transcriptional control of the involucrin promoter. Optimal conditions for production of pseudotyped VSV-G viral particles destined to transduce basal epithelial stem cells at the mucosal sites of entry of SIV in our animal model were determined. Altogether, the data demonstrate the feasibility of an epithelium-based vaccine containing involucrin-driven viral antigen encoding sequences that integrate into epithelial stem cells and show long-term expression in the upper layer of the epithelium even after multiple cycle of epithelia renewal. Such epithelium-based vaccine should elicit a long-term immunity against HIV/SIV infection at the site of entry of the virus.

Establishment of a neonatal rhesus macaque model to study Mycobacterium tuberculosis infection.

Mycobacterium tuberculosis (Mtb) is the causative agent of human tuberculosis (TB) with an estimated 8.8 million new TB cases and 1.4 million deaths annually. Tuberculosis is the leading cause of death in AIDS patients worldwide but very little is known about early TB infection or TB/HIV co-infection in infants. A clinically relevant newborn animal model to study TB infection is urgently needed. We have successfully established an aerosol newborn/infant model in neonatal nonhuman primates (NHPs) that mimics clinical and bacteriological characteristics of Mtb infection as seen in human newborns/infants. Further, this model will allow the establishment of a TB coinfection model of pediatric AIDS. Aerosol versus intra broncho-alveolar Mtb infection was studied. Interestingly, 42 days post infection specific lesions were detected suggestive of the classic Ghon focus in human children. Concurrently, specific cellular immune responses developed 4-6 weeks after Mtb infection. Using the enzyme-linked immunospot (ELISPOT) assays, we found that IL-12 production correlated with early Mtb infection lesions seen by routine thoracic radiographs. Overall, this work represents the first example of early Mtb infection of newborn macaques. This study gives us a unique opportunity to further characterize immunopathogenesis and establish a TB/SIV co-infection model for pediatric AIDS.

High cell-free virus load and robust autologous humoral immune responses in breast milk of simian immunodeficiency virus-infected african green monkeys.

The design of immunologic interventions to prevent postnatal transmission of human immunodeficiency virus (HIV) will require identification of protective immune responses in this setting. Simian immunodeficiency virus (SIV)-infected rhesus monkeys (RMs), a species that develops an AIDS-like illness following experimental infection, transmit the virus at a high rate during breastfeeding. In contrast, postnatal transmission of SIV occurs rarely or not at all in natural, asymptomatic primate hosts of SIV. These contrasting transmission patterns provide a unique opportunity to study mechanisms that evolved to protect suckling infants from SIV infection. We compared the virologic and immunologic properties of milk of SIV-infected and uninfected natural hosts of SIV, African green monkeys (AGMs), to that of RMs. Interestingly, despite a low number of milk CD4(+) T lymphocytes in uninfected AGMs, milk virus RNA load in SIV-infected AGMs was comparable to that of SIV-infected RMs and that in AGM plasma. This observation is in contrast to the relatively low virus load in milk compared to that in plasma of SIV-infected RMs and HIV-infected women. Milk of SIV-infected AGMs also displayed robust virus-specific cellular immune responses. Importantly, an autologous challenge virus-specific neutralization response was detected in milk of five of six SIV-infected AGMs that was comparable in magnitude to that in plasma. In contrast, autologous challenge virus neutralization was not detectable in milk of SIV-infected RMs. The autologous virus-specific adaptive immune responses in breast milk of AGMs may contribute to impedance of virus transmission in the infant oral/gastrointestinal tract and the rarity of postnatal virus transmission in natural hosts of SIV.

Vaccine protection by live, attenuated simian immunodeficiency virus in the absence of high-titer antibody responses and high-frequency cellular immune responses measurable in the periphery.

An attenuated derivative of simian immunodeficiency virus strain 239 deleted of V1-V2 sequences in the envelope gene (SIV239DeltaV1-V2) was used for vaccine/challenge experiments in rhesus monkeys. Peak levels of viral RNA in plasma of 10(4) to 10(6.5) copies/ml in the weeks immediately following inoculation of SIV239DeltaV1-V2 were 10- to 1,000-fold lower than those observed with parental SIV239 ( approximately 10(7.3) copies/ml). Viral loads consistently remained below 200 copies/ml after 8 weeks of infection by the attenuated SIV239DeltaV1-V2 strain. Viral localization experiments revealed large numbers of infected cells within organized lymphoid nodules of the colonic gut-associated lymphoid tissue at 14 days; double-labeling experiments indicated that 93.5% of the virally infected cells at this site were positive for the macrophage marker CD68. Cellular and humoral immune responses measured principally by gamma interferon enzyme-linked immunospot and neutralization assays were variable in the five vaccinated monkeys. One monkey had responses in these assays comparable to or only slightly less than those observed in monkeys infected with parental, wild-type SIV239. Four of the vaccinated monkeys, however, had low, marginal, or undetectable responses in these same assays. These five vaccinated monkeys and three naïve control monkeys were subsequently challenged intravenously with wild-type SIV239. Three of the five vaccinated monkeys, including the one with strong anti-SIV immune responses, were strongly protected against the challenge on the basis of viral load measurements. Surprisingly, two of the vaccinated monkeys were strongly protected against SIV239 challenge despite the presence of cellular anti-SIV responses of low-frequency and low-titer anti-SIV antibody responses. These results indicate that high-titer anti-SIV antibody responses and high-frequency anti-SIV cellular immune responses measurable by standard assays from the peripheral blood are not needed to achieve strong vaccine protection, even against a difficult, neutralization-resistant strain such as SIV239.

Induction of a virus-specific effector-memory CD4+ T cell response by attenuated SIV infection.

We investigated simian immunodeficiency virus (SIV)-specific CD4+ T cell responses in rhesus macaques chronically infected with attenuated or pathogenic SIV strains. Analysis of SIVDeltanef-infected animals revealed a relatively high frequency of SIV-specific CD4+ T cells representing 4-10% of all CD4+ T lymphocytes directed against multiple SIV proteins. Gag-specific CD4+ T cells in wild-type SIV-infected animals were 5-10-fold lower in frequency and inversely correlated with the level of plasma viremia. SIV-specific CD4+ cells from SIVDeltanef animals were predominantly CD27-CD28-CD45RAlowCCR7-CCR5-, consistent with an effector-memory subset, and included a fully differentiated CD45RA+CCR7- subpopulation. In contrast, SIV-specific CD4+ T cells from SIV-infected animals were mostly CD27+CD28+CD45RA-CCR7+CCR5+, consistent with an early central memory phenotype. The CD45RA+CCR7-CD4+ subset from SIVDeltanef animals was highly enriched for effector CD4+ T cells, as indicated by the perforin expression and up-regulation of the lysosomal membrane protein CD107a after SIV Gag stimulation. SIV-specific CD4+ T cells in attenuated SIV-infected animals were increased in frequency in bronchioalveolar lavage and decreased in lymph nodes, consistent with an effector-memory T cell population. The ability of SIVDeltanef to induce a high frequency virus-specific CD4+ T cell response with direct effector function may play a key role in protective immunity produced by vaccination with attenuated SIV strains.

Expression of CD8alpha identifies a distinct subset of effector memory CD4+ T lymphocytes.

Circulating CD4+ CD8+ T lymphocytes have been described in the peripheral blood of humans and several animal species. However, the origin and functional properties of these cells remain poorly understood. In the present study, we evaluated the frequency, phenotype and function of peripheral CD4+ CD8+ T cells in rhesus macaques. Two distinct populations of CD4+ CD8+ T cells were identified: the dominant one was CD4hi CD8lo and expressed the CD8alphaalpha homodimer, while the minor population was CD4lo CD8hi and expressed the CD8alphabeta heterodimer. The majority of CD4hi CD8alphalo T cells exhibited an activated effector/memory phenotype (CCR5lo CD7- CD28- HLA-DR+) and expressed relatively high levels of granzyme B. Intracellular cytokine staining assays demonstrated that the frequency of cytomegalovirus-specific T cells was enriched five-fold in CD4hi CD8alphalo T cells compared to single-positive CD4+ T cells, whereas no consistent enrichment was observed for simian immunodeficiency virus (SIV)-specific T cells. Cross-sectional studies of SIV-infected animals demonstrated that the frequency of CD4hi CD8alphalo T cells was lower in wild-type SIV-infected animals compared to uninfected controls, although prospective studies of SIV-infected animals demonstrated depletion of CD4hi CD8alphalo lymphocytes only in a subset of animals. Taken together, these data suggest that CD4+ T cells expressing CD8alpha represent an effector/memory subset of CD4+ T cells and that this cell population can be depleted during the course of SIV infection.

Intracellular cytokine staining for the characterization and quantitation of antigen-specific T lymphocyte responses.

Standard proliferation assays used for analysis of T cell function have significant shortcomings, including limited sensitivity, lack of quantitative readouts, and considerable variability. Recently, flow cytometric methods have been developed to allow multiparametric detection of cell surface antigens and intracellular cytokine expression in response to polyclonal stimuli and antigen. We have optimized an intracellular cytokine staining assay in the non-human primate model of AIDS, which allows us to identify antigen-specific T lymphocytes at the single cell level with high sensitivity, while reducing background staining to a minimum. Central to our optimized protocol is the addition of cross-linked costimulatory anti-CD28 and anti-CD49d Mabs, a modification that results in up to 3-fold enhancement of the frequency of cytokine-secreting CD4(+) T cells following superantigen or antigen-specific stimulation. Optimization of the antigen concentration and duration of antigenic stimulation resulted in a convenient and highly reproducible assay, which permits delineation of antigen-specific cells at the single cell level, thereby providing new insights into pathogen-specific immune responses and allowing detailed phenotypic analysis of extremely low frequency events.

Optimization of intracellular cytokine staining for the quantitation of antigen-specific CD4+ T cell responses in rhesus macaques.

Standard proliferation assays used for analysis of CD4+ T cell function have significant shortcomings, including limited sensitivity, lack of truly quantitative readouts and significant variability. We have optimized an intracellular cytokine staining (ICS) assay in rhesus macaques which allows us to identify virus-specific CD4+ T cells at the single-cell level with high sensitivity while reducing background staining to a minimum. A variety of parameters were tested to determine the optimal experimental conditions necessary for the detection of antigen-specific CD4+ T cells in macaques. Central to our optimized protocol was the addition of cross-linked costimulatory anti-CD28 and anti-CD49d Mabs, a modification which resulted in up to threefold enhancement of the frequency of TNF-alpha-secreting CD4+ T cells following superantigen- or antigen-specific stimulation. The ICS protocol was also optimized with respect to antigen concentration and duration of antigenic stimulation. These modifications resulted in a convenient and highly reproducible assay with intra- and inter-assay variability of less than 10%. Although cryopreservation of PBMC generally led to a 40% to 80% decrease in the frequency of antigen-specific CD4+ T cells detected by ICS using stimulation with viral proteins, the use of overlapping peptide pools minimized the effects of cryopreservation on ICS responses. The use of more sensitive techniques such as ICS permits delineation of antigen-specific cells at the single cell level and should provide new insights into pathogen-specific immune responses in the rhesus macaque model.

Elevated interleukin-7 levels not sufficient to maintain T-cell homeostasis during simian immunodeficiency virus-induced disease progression.

Elevated levels of interleukin 7 (IL-7) have been correlated with various T-cell depletion conditions, including HIV infection, and suggested as an indicator of HIV disease progression (AIDS and death). Here, the assessment of pathogenic simian immunodeficiency virus (SIVmac239) infection in rhesus macaques demonstrated a clear association between a significant elevation in IL-7 levels and disease progression. In 5 macaques that progressed to simian AIDS and death, elevated IL-7 levels were unable to restore T-cell homeostasis. In contrast, increased IL-7 levels were followed by relatively high and stable T-cell numbers in the SIV-infected macaques with a slow-progressing phenotype. Further, studies in sooty mangabeys that do not progress to simian AIDS and that maintain stable T-cell numbers despite high levels of viral replication support the importance of IL-7 and T-cell homeostasis in disease progression. These data suggest that during pathogenic SIV infection with high viral replication, elevated IL-7 levels are unable to recover T-cell homeostasis, thereby leading to disease progression. The utility of IL-7 as a potential immunotherapeutic agent to improve HIV/SIV-related T-cell depletion may therefore depend on controlling the pathogenic effects of viral replication prior to the administration of IL-7.