Impaired T-cell differentiation in the thymus at the early stages of acute pathogenic chimeric simian-human immunodeficiency virus (SHIV) infection in contrast to less pathogenic SHIV infection

One of the mechanisms by which HIV infection induces the depletion of CD4+ T cells has been suggested to be impairment of T-cell development in the thymus, although there is no direct evidence that this occurs. To examine this possibility, we compared T-cell maturation in the intrathymic progenitors between macaques infected with an acute pathogenic chimeric simian-human immunodeficiency virus (SHIV), which causes profound and irreversible CD4+ T-cell depletion, and macaques infected with a less pathogenic SHIV, which causes only a transient CD4+ T-cell decline. Within 27 days post-inoculation (dpi), the two virus infections caused similar increases in plasma viral loads and similar decreases in CD4+ T-cell counts. However, in the thymus, the acute pathogenic SHIV resulted in increased thymic involution, atrophy and the depletion of immature T cells including CD4(+)CD8(+) double-positive (DP) cells, whereas the less pathogenic SHIV did not have these effects. Ex vivo differentiation of CD3(-)CD4(-)CD8(-) triple-negative (TN) intrathymic progenitors to DP cells was assessed by a monkey-mouse xenogenic fetal thymus organ culture system. Differentiation was impaired in the TN intrathymic progenitors of the acute pathogenic SHIV-infected monkeys, while differentiation was not impaired in the TN intrathymic progenitors of the less pathogenic SHIV-infected monkeys. These differences suggest that dysfunction of thymic maturation makes an important contribution to the irreversible depletion of circulating CD4+ T cells in vivo.     

Early virological events in various tissues of newborn monkeys after intrarectal infection with pathogenic simian human immunodeficiency virus

Children infected with human immunodeficiency virus type 1 often have higher viral loads and progress to acquired immunodeficiency syndrome more rapidly than adults. In our previous study of simian-human immunodeficiency virus (SHIV)-infected adult monkeys, immature CD4CD8 double-positive T cells in the thymus and jejunum decreased faster than mature CD4 single-positive T cells. Here, we examined the effect of virus replication on immature T cells from the same SHIV-inoculated newborn monkeys having more immature T cells than adults. The infectious viruses were more abundantly detected in the thymus than in other tissues at both 13 and 26 days post-infection (dpi). However, mature CD4(+) T cells in the thymus declined after 13 dpi and immature CD3(-) CD4 single-positive T cells remained at 26 dpi. These results suggested that many immature CD4(+) T cells in the thymus of newborns support the production of infectious viruses even after the depletion of mature CD4(+) T cells.     

Simian immunodeficiency virus infection in neonatal macaques

Children with human immunodeficiency virus infection often have higher viral loads and progress to AIDS more rapidly than adults. Since the intestinal tract is a major site of early viral replication and CD4(+) T-cell depletion in adults, we examined the effects of simian immunodeficiency virus (SIV) on both peripheral and intestinal lymphocytes from 13 neonatal macaques infected with SIVmac239. Normal neonates had more CD4(+) T cells and fewer CD8(+) T cells in all tissues than adults. Surprisingly, neonates had substantial percentages of CD4(+) T cells with an activated, memory phenotype (effector CD4(+) T cells) in the lamina propria of the intestine compared to peripheral lymphoid tissues, even when examined on the day of birth. Moreover, profound and selective depletion of jejunum lamina propria CD4(+) T cells occurred in neonatal macaques within 21 days of infection, which was preceded by large numbers of SIV-infected cells in this compartment. Furthermore, neonates with less CD4(+) T-cell depletion in tissues tended to have higher viral loads. The persistence of intestinal lamina propria CD4(+) T cells in some neonates with high viral loads suggests that increased turnover and/or resistance to CD4(+) T-cell loss may contribute to the higher viral loads and increased severity of disease in neonatal hosts.     

Generation of CD3+CD8low thymocytes in the HIV type 1-infected thymus

Infection with the HIV type 1 (HIV-1) can result both in depletion of CD4(+) T cells and in the generation of dysfunctional CD8(+) T cells. In HIV-1-infected children, repopulation of the peripheral T cell pool is mediated by the thymus, which is itself susceptible to HIV-1 infection. Previous work has shown that MHC class I (MHC I) molecules are strongly up-regulated as result of IFN-alpha secretion in the HIV-1-infected thymus. We demonstrate in this study that increased MHC I up-regulation on thymic epithelial cells and double-positive CD3(-/int)CD4(+)CD8(+) thymocytes correlates with the generation of mature single-positive CD4(-)CD8(+) thymocytes that have low expression of CD8. Treatment of HIV-1-infected thymus with highly active antiretroviral therapy normalizes MHC I expression and surface CD8 expression on such CD4(-)CD8(+) thymocytes. In pediatric patients with possible HIV-1 infection of the thymus, a low CD3 percentage in the peripheral circulation is also associated with a CD8(low) phenotype on circulating CD3(+)CD8(+) T cells. Furthermore, CD8(low) peripheral T cells from these HIV-1(+) pediatric patients are less responsive to stimulation by Ags from CMV. These data indicate that IFN-alpha-mediated MHC I up-regulation on thymic epithelial cells may lead to high avidity interactions with developing double-positive thymocytes and drive the selection of dysfunctional CD3(+)CD8(low) T cells. We suggest that this HIV-1-initiated selection process may contribute to the generation of dysfunctional CD8(+) T cells in HIV-1-infected patients.     

T-cell receptor excision circles (TREC) in SHIV 89.6p and SIVmac251 models of HIV-1 infection

T-cell receptor excision circles (TREC) may be a useful surrogate marker in HIV-1 infection for evaluating the likelihood of continued clinical stability and/or the response to therapeutics, including vaccines. Analysis of TREC in SHIV and SIV models of HIV-1 infection may provide additional information concerning the utility of TREC as a marker. We measured TREC in peripheral blood mononuclear cells (PBMC) from rhesus macaques in SHIV89.6p (n = 20) and SIVmac251 (n = 11) models of HIV-1 infection. TREC were also evaluated in tissues in the SIVmac251 model at end-point. In the SHIV89.6p model, TREC in PBMC were significantly lower at 12 weeks postinfection compared to preinfection levels. The decrease in TREC correlated with the decline in CD4+ T cells (r(s) = 0.496; P = 0.026), which in turn correlated inversely with serum viral loads at end-point (r(s) = -0.517; P = 0.019). Macaques that controlled SHIV89.6p infection to some degree (n = 6) had higher TREC at study end-point (P = 0.017). In the SIVmac251 model, TREC in PBMC were significantly reduced after 17 months of infection (P = 0.012) despite receiving highly active antiretroviral therapy (HAART) consisting of didanosine (ddI) and (R)-9-(2-phosphonylmethoxypropyl)-adenine (PMPA) when not cycling off therapy during scheduled treatment interruptions (STI). However, macaques that received continuous hydroxyurea (HU) in addition to the HAART regimen had higher end-point TREC compared to the non-HU group (P = 0.041), and the reduction in TREC observed at end-point within the HU group was not significant. In the SIVmac251 model, TREC correlated with the percentage of CD4+ T cells (r(s) = 0.426; P = 0.048) and CD4+CD28+ T cells (r(s) = 0.624; P = 0.002), and inversely with CD8+ T cells (r(s) = -0.622; P = 0.002), CD8+CD28- T cells (r(s) = -0.516; P = 0.014), and serum viral loads (r(s) = -0.627; P = 0.039). High levels of TREC were observed in the thymus, levels comparable to PBMC were seen in the lymph node, and low but detectable levels of TREC were present in bone marrow. The use of correlates of TREC as covariates in ANCOVA revealed that the decline in TREC in the SHIV 89.6p model reflected the decline in the percentage of CD4+ T-cells due to viral cytopathogenicity. In the SIVmac251 model, the decline in TREC was related to increased immune activation and proliferation due to viral replication, as reflected by decreases in percentages of CD4+CD28+ T cells and increases in CD8+ and CD8+CD28- T cells.     

Enhanced infectivity of an R5-tropic simian/human immunodeficiency virus carrying human immunodeficiency virus type 1 subtype C envelope after serial passages in pig-tailed macaques (Macaca nemestrina)

The increasing prevalence of human immunodeficiency virus type 1 (HIV-1) subtype C infection worldwide calls for efforts to develop a relevant animal model for evaluating strategies against the transmission of the virus. A chimeric simian/human immunodeficiency virus (SHIV), SHIV(CHN19), was generated with a primary, non-syncytium-inducing HIV-1 subtype C envelope from a Chinese strain in the background of SHIV(33). Unlike R5-tropic SHIV(162), SHIV(CHN19) was not found to replicate in rhesus CD4(+) T lymphocytes. SHIV(CHN19) does, however, replicate in CD4(+) T lymphocytes of pig-tailed macaques (Macaca nemestrina). The observed replication competence of SHIV(CHN19) requires the full tat/rev genes and partial gp41 region derived from SHIV(33). To evaluate in vivo infectivity, SHIV(CHN19) was intravenously inoculated, at first, into two pig-tailed and two rhesus macaques. Although all four animals became infected, the virus replicated preferentially in pig-tailed macaques with an earlier plasma viral peak and a faster seroconversion. To determine whether in vivo adaptation would enhance the infectivity of SHIV(CHN19), passages were carried out serially in three groups of two pig-tailed macaques each, via intravenous blood-bone marrow transfusion. The passages greatly enhanced the infectivity of the virus as shown by the increasingly elevated viral loads during acute infection in animals with each passage. Moreover, the doubling time of plasma virus during acute infection became much shorter in passage 4 (P4) animals (0.2 day) in comparison to P1 animals (1 to 2 days). P2 to P4 animals all became seropositive around 2 to 3 weeks postinoculation and had a decline in CD4/CD8 T-cell ratio during the early phase of infection. In P4 animals, a profound depletion of CD4 T cells in the lamina propria of the jejunum was observed. Persistent plasma viremia has been found in most of the infected animals with sustained viral loads ranging from 10(3) to 10(5) per ml up to 6 months postinfection. Serial passages did not change the viral phenotype as confirmed by the persistence of the R5 tropism of SHIV(CHN19) isolated from P4 animals. In addition, the infectivity of SHIV(CHN19) in rhesus peripheral blood mononuclear cells was also increased after in vivo passages. Our data indicate that SHIV(CHN19) has adapted well to grow in macaque cells. This established R5-tropic SHIV(CHN19)/macaque model would be very useful for HIV-1 subtype C vaccine and pathogenesis studies.     

Viral infection of the thymus.

We have examined infection of the thymus during congenitally acquired chronic lymphocytic choriomeningitis virus (LCMV) infection of mice, a classic model of antigen-specific T-cell tolerance. Our results show that (i) infection starts at the fetal stage and is maintained throughout adulthood, and (ii) this chronic infection of the thymus can be eliminated by transfer of virus-specific cytotoxic T lymphocytes (CTL) that infiltrate the thymus and clear all viral products from both medullary and cortical regions. Elimination of virus from the thymus results in abrogation of tolerance. During the fetal stage, the predominant cell type infected is the earliest precursor of T cells with a surface phenotype of Thy1+ CD4- CD8- J11d+. In the adult thymus, infection is confined primarily to the cortisone-resistant thymocytes present in the medullary region. The infected cells are CD4+ and J11d+. The presence of J11d, a marker usually associated with immature thymocytes, on infected single positive CD4+ "mature" thymocytes is intriguing and suggests that infection by this noncytolytic virus may affect development of T cells. There is minimal infection of the CD8+ medullary thymocytes or of the double positive (CD4+ CD8+) cells present in the cortex. Infection within the cortex is confined to the stromal cells. Interestingly, there is infection of the double negative (CD4- CD8-) thymocytes in the adult thymus, showing that even during adulthood the newly developing T cells are susceptible to infection by LCMV. Virus can be eliminated from the thymuses of these carrier mice by adoptive transfer of medullary region first and then from the thymic cortex. This result clearly shows the need to reevaluate the widely held notion that mature T cells are unable to reenter the thymus. In fact, in our experiments the donor T cells made up to 20 to 30% of the total cells in the thymus at 5 to 7 days after the transfer. The number of donor T cells declined as virus was eliminated from the thymus, and at 1 month posttransfer, the donor T cells were hardly detectable. The results of this study examining the dynamics of viral infection and clearance from the thymus, the primary site of T-cell development, have implications for understanding tolerance induction in chronic viral infections.     

In vivo treatment of class II MHC-deficient mice with anti-TCR antibody restores the generation of circulating CD4 T cells and optimal architecture of thymic medulla

TCR ligation by the self-peptide-associated MHC molecules is essential for T cell development in the thymus, so that class II MHC-deficient mice do not generate CD4(+)CD8(-) T cells. The present results show that the administration of anti-TCR mAb into class II MHC-deficient mice restores the generation of CD4(+)CD8(-) T cells in vivo. The CD4 T cells were recovered in the thymus, peripheral blood, and the spleen, indicating that the anti-TCR treatment is sufficient for peripheral supply of newly generated CD4 T cells. Unlike peripheral CD4 T cells that disappeared within 5 wk after the treatment, CD4(+)CD8(-) thymocytes remained undiminished even after 5 wk, suggesting that CD4 T cells in the thymus are maintained separately from circulating CD4 T cells and even without class II MHC molecules. It was also found that the mass of medullary region in the thymus, which was reduced in class II MHC-deficient mice, was restored by the anti-TCR administration, suggesting that the medulla for CD4(+)CD8(-) thymocytes is formed independently of the medulla for CD4(-)CD8(+) thymocytes. These results indicate that in vivo anti-TCR treatment in class II MHC-deficient mice restores the generation of circulating CD4 T cells and optimal formation of the medulla in the thymus, suggesting that anti-TCR Ab may be useful for clinical treatment of class II MHC deficiencies.     

TCR gamma delta+ and CD161+ thymocytes express HIV-1 in the SCID-hu mouse,potentially contributing to immune dysfunction in HIV infection

The vast diversity of the T cell repertoire renders the adaptive immune response capable of recognizing a broad spectrum of potential antigenic peptides. However, certain T cell rearrangements are conserved for recognition of specific pathogens, as is the case for TCRgammadelta cells. In addition, an immunoregulatory class of T cells expressing the NK receptor protein 1A (CD161) responds to nonpeptide Ags presented on the MHC-like CD1d molecule. The effect of HIV-1 infection on these specialized T cells in the thymus was studied using the SCID-hu mouse model. We were able to identify CD161-expressing CD3(+) cells but not the CD1d-restricted invariant Valpha24/Vbeta11/CD161(+) NK T cells in the thymus. A subset of TCRgammadelta cells and CD161-expressing thymocytes express CD4, CXCR4, and CCR5 during development in the thymus and are susceptible to HIV-1 infection. TCRgammadelta thymocytes were productively infectable by both X4 and R5 virus, and thymic HIV-1 infection induced depletion of CD4(+) TCRgammadelta cells. Similarly, CD4(+)CD161(+) thymocytes were depleted by thymic HIV-1 infection, leading to enrichment of CD4(-)CD161(+) thymocytes. Furthermore, compared with the general CD4-negative thymocyte population, CD4(-)CD161(+) NK T thymocytes exhibited as much as a 27-fold lower frequency of virus-expressing cells. We conclude that HIV-1 infection and/or disruption of cells important in both innate and acquired immunity may contribute to the overall immune dysfunction seen in HIV-1 disease.     

Activated Memory CD4+ T Helper Cells Repopulate the Intestine Early following Antiretroviral Therapy of Simian Immunodeficiency Virus-Infected Rhesus Macaques but Exhibit a Decreased Potential To Produce Interleukin-2

Using the simian immunodeficiency virus (SIV)-infected rhesus macaque model, we performed a longitudinal study to determine the effect of antiretroviral therapy on the phenotype and functional potential of CD4(+) T cells repopulating intestinal mucosa in human immunodeficiency virus infection. Severe depletion of CD4(+) and CD4(+) CD8(+) T cells occurred in the intestinal mucosa during primary SIV infection. The majority of these cells were of activated memory phenotype. Phosphonate 9-[2-(phosphomethoxypropyl]adenine (PMPA) treatment led to a moderate suppression of intestinal viral loads and repopulation of intestinal mucosa by predominantly activated memory CD4(+) T-helper cells. This repopulation was independent of the level of viral suppression. Compared to preinfection values, the frequency of naive CD4(+) T cells increased following PMPA therapy, suggesting that new CD4(+) T cells were repopulating the intestinal mucosa. Repopulation by CD4(+) CD8(+) T cells was not observed in either jejunum or colon lamina propria. The majority of CD4(+) T cells repopulating the intestinal mucosa following PMPA therapy were CD29(hi) and CD11ahi. A subset of repopulating intestinal CD4(+) T cells expressed Ki-67 antigen, indicating that local proliferation may play a role in the repopulation process. Although the majority of repopulating CD4(+) T cells in the intestinal mucosa were functionally capable of providing B- and T-cell help, as evidenced by their expression of CD28, these CD4(+) T cells were found to have a reduced capacity to produce interleukin-2 (IL-2) compared to the potential of CD4(+) T cells prior to SIV infection. Persistent viral infection may play a role in suppressing the potential of repopulating CD4(+) T cells to produce IL-2. Hence, successful antiretroviral therapy should aim at complete suppression of viral loads in mucosal lymphoid tissues, such as intestinal mucosa.     

A role of CXC chemokine ligand 12/stromal cell-derived factor-1/pre-B cell growth stimulating factor and its receptor CXCR4 in fetal and adult T cell development in vivo

The functions of a chemokine CXC chemokine ligand (CXCL) 12/stromal cell-derived factor-1/pre-B cell growth stimulating factor and its physiologic receptor CXCR4 in T cell development are controversial. In this study, we have genetically further characterized their roles in fetal and adult T cell development using mutant and chimeric mice. In CXCL12(-/-) or CXCR4(-/-) embryos on a C57BL/6 background, accumulation of T cell progenitors in the outer mesenchymal layer of the thymus anlage during initial colonization of the fetal thymus was comparable with that seen in wild-type embryos. However, the expansion of CD3(-)CD4(-)CD8(-) triple-negative T cell precursors at the CD44(-)CD25(+) and CD44(-)CD25(-) stages, and CD4(+)CD8(+) double-positive thymocytes was affected during embryogenesis in these mutants. In radiation chimeras competitively repopulated with CXCR4(-/-) fetal liver cells, the reduction in donor-derived thymocytes compared with wild-type chimeras was much more severe than the reduction in donor-derived myeloid lineage cells in bone marrow. Triple negative CD44(+)CD25(+) T cell precursors exhibited survival response to CXCL12 in the presence of stem cell factor as well as migratory response to CXCL12. Thus, it may be that CXCL12 and CXCR4 are involved in the expansion of T cell precursors in both fetal and adult thymus in vivo. Finally, enforced expression of bcl-2 did not rescue impaired T cell development in CXCR4(-/-) embryos or impaired reconstitution of CXCR4(-/-) thymocytes in competitively repopulated mice, suggesting that defects in T cell development caused by CXCR4 mutation are not caused by reduced expression of bcl-2.     

Subtle defects in pre-TCR signaling in the absence of the Tec kinase Itk

alphabeta T cell development in the thymus is dependent on signaling through the TCR. The first of these signals is mediated by the pre-TCR, which is responsible for promoting pre-T cell proliferation and the differentiation of CD4(-)8(-)3(-) (DN) thymocytes into CD4(+)8(+)3(+) (DP) cells. In many cases, T cell signaling proteins known to be essential for TCR signaling in mature T cells are also required for pre-TCR signaling in DN thymocytes. Therefore, it came as a surprise to discover that mice lacking the Tec kinases Itk and Rlk, enzymes required for efficient activation of phospholipase C-gamma1 in mature T cells, showed no obvious defects in pre-TCR-dependent selection events in the thymus. In this report, we demonstrate that DN thymocytes lacking Itk, or Itk and Rlk, are impaired in their ability to generate normal numbers of DP thymocytes, especially when placed in direct competition with WT DN thymocytes. We also show that Itk is required for maximal pre-TCR signaling in DN thymocytes. These data demonstrate that the Tec kinases Itk and Rlk are involved in, but are not essential for, pre-TCR signaling in the thymus, suggesting that there is an alternative mechanism for activating phospholipase C-gamma1 in DN thymocytes that is not operating in DP thymocytes and mature T cells.     

Mechanism of human immunodeficiency virus type 1 localization in CD4-negative thymocytes: differentiation from a CD4-positive precursor allows productive infection.

Human immunodeficiency virus (HIV) infection of the thymus could have profound effects on development of the immune response, particularly in children. We and others have established that in addition to infecting and depleting CD4-bearing thymocytes, functional HIV proviruses are found in thymocytes lacking surface CD4 expression. Using in vitro thymocyte cultures, we show that neither HIV-mediated down regulation of CD4 nor CD4-independent infection contributes to the localization of HIV in cells lacking the primary virus receptor. Rather, infection of a CD4-positive precursor cell (CD4 positive/CD8 positive) with subsequent differentiation into a mature CD4-negative phenotype results in productively infected CD4-negative cells. This novel mechanism may contribute to pathogenesis by distributing viral sequences into functional subsets of T cells typically refractory to HIV infection and could account for the presence of viral DNA in CD8-positive lymphocytes recently observed in patients.     

The transient expression of C-C chemokine receptor 8 in thymus identifies a thymocyte subset committed to become CD4+ single-positive T cells

Developing T cells journey through the different thymic microenvironments while receiving signals that eventually will allow some of them to become mature naive T cells exported to the periphery. This maturation can be visualized by the phenotype of the developing cells. CCR8 is a ss-chemokine receptor preferentially expressed in the thymus. We have developed 8F4, an anti-mouse CCR8 mAb that is able to neutralize the ligand-induced activation of CCR8, and used it to characterize the CCR8 protein expression in the different thymocyte subsets. Taking into account the intrathymic lineage relationships, our data showed that CCR8 expression in thymus followed two transient waves along T cell maturation. The first one took place in CD4(-) CD8(-) double-negative thymocytes, which showed a low CCR8 expression, and the second wave occurred after TCR activation by the Ag-dependent positive selection in CD4(+) CD8(+) double-positive cells. From that maturation stage, CCR8 expression gradually increased as the CD4(+) cell differentiation proceeded, reaching a maximum at the CD4(+) CD8(-) single-positive stage. These CD4(+) cells expressing CCR8 were also CD69(high) CD62L(low) thymocytes, suggesting that they still needed to undergo some differentiation step before becoming functionally competent naive T cells ready to be exported from the thymus. Interestingly, no significant amounts of CCR8 protein were detectable in CD4(-) CD8(+) thymocytes. Our data showing a clear regulation of the CCR8 protein in thymus suggest a relevant role for CCR8 in this lymphoid organ, and identify CCR8 as a possible marker of thymocyte subsets recently committed to the CD4(+) lineage.     

Experimental Trypanosoma cruzi infection alters the shaping of the central and peripheral T-cell repertoire

We investigated the thymic and peripheral T-lymphocyte subsets in BALB/c mice undergoing acute or chronic Trypanosoma cruzi infection, in terms of expression of particular Vbeta rearrangements of the T-cell receptor. We first confirmed the severe depletion of CD4(+)CD8(+) thymocytes following acute T. cruzi infection. By contrast, the numbers of CD4(+)CD8(+) cells in subcutaneous lymph nodes increased up to 16 times. In subcutaneous lymph nodes, we found CD4(+)CD8(+) cells that expressed prohibited segments TCRVbeta5 and TCRVbeta12 (which are physiologically deleted in the thymus of BALB/c mice), as did some mature single-positive cells (CD4(+)CD8(-) and CD4(-)CD8(+)). In the thymus of infected animals, although higher numbers of immature cells bearing such Vbeta segments were seen, they were no longer detected in the mature single-positive stage, suggesting that negative selection occurs normally. We also found increased numbers of cells bearing the potentially autoreactive phenotype TCRVbeta5(+) and TCRVbeta12(+) in T-lymphocyte subsets from subcutaneous lymph nodes of T. cruzi chronically infected mice. In conclusion, our data indicate that immature T lymphocytes bearing prohibited TCRVbeta segments leave the thymus and gain the lymph nodes, where they further differentiate into mature CD4(+) or CD8(+) cells. Conjointly, these findings show changes in the shaping of the central and peripheral T-cell repertoire in both acute and chronic phases of murine T. cruzi infection. The release of potentially autoreactive T cells in the periphery of the immune system may contribute to the autoimmune process found in both murine and human Chagas' disease.     

Highly effective control of an AIDS virus challenge in macaques by using vesicular stomatitis virus and modified vaccinia virus Ankara vaccine vectors in a single-boost protocol

Previous studies have shown that vaccination and boosting of rhesus macaques with attenuated vesicular stomatitis virus (VSV) vectors encoding Env and Gag proteins of simian immunodeficiency virus-human immunodeficiency virus (SHIV) hybrid viruses protect rhesus macaques from AIDS after challenge with the highly pathogenic SHIV 89.6P (23). In the present study, we compared the effectiveness of a single prime-boost protocol consisting of VSV vectors expressing SHIV Env, Gag, and Pol proteins to that of a protocol consisting of a VSV vector prime followed with a single boost with modified vaccinia virus Ankara (MVA) expressing the same SHIV proteins. After challenge with SHIV 89.6P, MVA-boosted animals controlled peak challenge viral loads to less than 2 x 10(6) copies/ml (a level significantly lower than that seen with VSV-boosted animals and lower than those reported for other vaccine studies employing the same challenge). MVA-boosted animals have shown excellent preservation of CD4(+) T cells, while two of four VSV-boosted animals have shown significant loss of CD4(+) T cells. The improved protection in MVA-boosted animals correlates with trends toward stronger prechallenge CD8(+)-T-cell responses to SHIV antigens and stronger postchallenge SHIV-neutralizing antibody production.     

Vaccination-induced noncytolytic effects in the acute phase of SHIV infection

Many studies have shown that vaccines inducing CD8+ T cell responses can reduce viral loads and preserve CD4+ T cell numbers in monkey models of HIV infection. The mechanism of viral control by the vaccine-induced CD8+ T cells is usually assumed to be cytolysis of infected cells. However, in addition to cytolysis of infected cells, CD8+ T cells secrete a range of soluble factors that suppress viral replication. We have studied the dynamics of virus and CD4+ T cells in a successful vaccination-challenge model of SHIV infection. We find that better viral control in the acute phase of infection is associated with slower decay of peak viral load. Comparing viral and CD4+ T cell dynamics in acute infection, we find that a cytolytic mode of viral control with direct killing of infected cells is inconsistent with the observed trends. On the other hand, comparison of the predicted effects of noncytolytic CD8+ effector function with the experimental data shows that non-cytolytic control provides a better explanation of the experimental results. Our analysis suggests that vaccine-induced CD8+ T cells control SHIV infection by non-cytolytic means.