Expression of herpes simplex virus ICP47 and human cytomegalovirus US11 prevents recognition of transgene products by CD8(+) cytotoxic T lymphocytes

The in vivo persistence of gene-modified cells may be limited by the development of a host immune response to vector-encoded proteins. Herpesviruses evade cytotoxic T-lymphocyte (CTL) recognition by expressing genes which interfere selectively with presentation of viral antigens by class I major histocompatibility complex (MHC) molecules. Here, we studied the use of retroviral vectors encoding herpes simplex virus ICP47, human cytomegalovirus (HCMV) US3, or HCMV US11 to decrease presentation of viral proteins and transgene products to CD8(+) CTL. Human fibroblasts and T cells transduced to express the ICP47, US3, or US11 genes alone exhibited a decrease in cell surface class I MHC expression. The combination of ICP47 and US11 rendered fibroblasts negative for surface class I MHC and allowed a class I MHC-low population of T cells to be sorted by flow cytometry. Fibroblasts and T cells expressing both ICP47 and US11 were protected from CTL-mediated lysis and failed to stimulate specific memory T-cell responses to transgene products in vitro. Our findings suggest that expression of immunoregulatory viral gene products could be a potential strategy to prolong transgene expression in vivo.     

A replication competent adenovirus 5 host range mutant-simian immunodeficiency virus (SIV) recombinant priming/subunit protein boosting vaccine regimen induces broad,persistent SIV-specific cellular immunity to dominant and subdominant epitopes in Mamu-A*01 rhesus macaques

CTL are important in controlling HIV and SIV infection. To quantify cellular immune responses induced by immunization, CD8(+) T cells specific for the subdominant Env p15m and p54m epitopes and/or the dominant Gag p11C epitope were evaluated by tetramer staining in nine macaques immunized with an adenovirus (Ad) 5 host range mutant (Ad5hr)-SIVenv/rev recombinant and in four of nine which also received an Ad5hr-SIVgag recombinant. Two Ad5hr-SIV recombinant priming immunizations were followed by two boosts with gp120 protein or an envelope polypeptide representing the CD4 binding domain. Two mock-immunized macaques served as controls. IFN-gamma-secreting cells were also assessed by ELISPOT assay using p11C, p15m, and p54m peptide stimuli and overlapping pooled Gag and Env peptides. As shown by tetramer staining, Ad-recombinant priming elicited a high frequency of persistent CD8(+) T cells able to recognize p11C, p15m, and p54m epitopes. The presence of memory cells 38 wk postinitial immunization was confirmed by expansion of tetramer-positive CD8(+) T cells following in vitro stimulation. The SIV-specific CD8(+) T cells elicited were functional and secreted IFN-gamma in response to SIV peptide stimuli. Although the level and frequency of response of peripheral blood CD8(+) T cells to the subdominant Env epitopes were not as great as those to the dominant p11C epitope, elevated responses were observed when lymph node CD8(+) T cells were evaluated. Our data confirm the potency and persistence of functional cellular immune responses elicited by replication competent Ad-recombinant priming. The cellular immunity elicited is broad and extends to subdominant epitopes.     

Vaccine-induced memory CD8+ T cells cannot prevent central nervous system virus reactivation

Noncytopathic viruses use multiple strategies to evade immune detection, challenging a role for vaccine induced CTL in preventing microbial persistence. Recrudescence of neurotropic coronavirus due to loss of T cell-mediated immune control provided an experimental model to test T cell vaccination efficacy in the absence of Ab. Challenge virus was rapidly controlled in vaccinated Ab-deficient mice coincident with accelerated recruitment of memory CD8+ T cells and enhanced effector function compared with primary CD8+ T cell responses. In contrast to primary effectors, reactivated memory cells persisted in the CNS at higher frequencies and retained ex vivo cytolytic activity. Nevertheless, despite earlier and prolonged T cell-mediated control in the CNS of vaccinated mice, virus ultimately reactivated. Apparent loss of memory CD8+ effector function in vivo was supported by a prominent decline in MHC expression on CNS resident target cells, presumably reflecting diminished IFN-gamma. Severely reduced MHC expression on glial cells at the time of recrudescence suggested that memory T cells, although fully armed to exert antiviral activity upon Ag recognition in vitro, are not responsive in an environment presenting few if any target MHC molecules. Paradoxically, effective clearance of viral Ag thus affords persisting virus a window of opportunity to escape from immune surveillance. These studies demonstrate that vaccine-induced T cell memory alone is unable to control persisting virus in a tissue with strict IFN-dependent MHC regulation, as evident in immune privileged sites.     

Immune tolerance to combined organ and bone marrow transplants after fractionated lymphoid irradiation involves regulatory NK T cells and clonal deletion

Immune tolerance to organ transplants has been reported in laboratory animals and in humans after nonmyeloablative conditioning of the host and infusion of donor bone marrow cells. We examined the mechanisms of immune tolerance to mouse cardiac allografts in MHC-mismatched hosts that developed mixed chimerism after posttransplant conditioning with a 2-wk course of multiple doses of lymphoid tissue irradiation, depletive anti-T cell Abs, and an infusion of donor bone marrow cells. When CD1(-/-) or J(alpha)281(-/-) hosts with markedly reduced NK T cells were used instead of wild-type hosts, then the conditioning regimen failed to induce tolerance to the heart allografts despite the development of mixed chimerism. Tolerance could be restored to the CD1(-/-) hosts by infusing enriched T cells from the bone marrow of wild-type mice containing CD1-reactive T cells but not from CD1(-/-) host-type mice. Tolerance could not be induced in either IL-4(-/-) or IL-10(-/-) hosts given the regimen despite the development of chimerism and clonal deletion of host T cells to donor MHC-Ags in the IL-10(-/-) hosts. We conclude that immune tolerance to bone marrow transplants involves clonal deletion, and tolerance to heart allografts in this model also involves regulatory CD1-reactive NK T cells.     

Escape mechanisms of melanoma from immune system by soluble melanoma antigen

We demonstrate in the B16 melanoma (C57BL/6 derived) system that the soluble form of tumor Ag preferentially suppresses immune responses 1) by inhibiting CTL activity in the effector phase and 2) by induction of specific Ts that block CTL generation in the induction phase. Soluble melanoma antigen Ag injected i.p. into the tumor-bearing host can effectively augment melanoma growth in vivo. Two T cell types with the L3T4+ or double-negative/I-J+ phenotype are involved in the suppression of anti-melanoma CTL responses and can easily be generated in the in vitro primary 12 h-culture. Anti-melanoma Ts recognizes the GM3(NeuAc) structure and distinguishes GM3 molecular species. This is because liposomes constructed with GM3(NeuAc) but not with GM3(NeuGc) gangliosides alone can effectively induce the melanoma-specific Ts. It is thus likely that tumor cells can escape from the immunologic surveillance system by stimulating the repertoire of Ts for self-Ag, GM3, which has existed even in the unprimed conditions in order to maintain self-tolerance. These would appear to be the major escape mechanisms.     

Early divergent host responses in SHIVsf162P3 and SIVmac251 infected macaques correlate with control of viremia

We previously showed intravaginal inoculation with SHIVsf162p3 results in transient viremia followed by undetectable viremia in most macaques, and some displayed subsequent immunity to superinfection with pathogenic SIVmac251. Here we compare early T cell activation, proliferation, and plasma cytokine/chemokine responses in macaques intravaginally infected with either SHIVsf162p3 or SIVmac251 to determine whether distinct differences in host responses may be associated with early viral containment. The data show SIVmac251 infection results in significantly higher levels of T cell activation, proliferation, and a mixed cytokine/chemokine "storm" in plasma in primary infection, whereas infection with SHIVsf162p3 resulted in significantly lower levels of T cell activation, proliferation, and better preservation of memory CD4+ T cells in early infection which immediately preceded control of viremia. These results support the hypothesis that early systemic immune activation, T cell proliferation, and a more prominent and broader array of cytokine/chemokine responses facilitate SIV replication, and may play a key role in persistence of infection, and the progression to AIDS. In contrast, immune unresponsiveness may be associated with eventual clearance of virus, a concept that may have key significance for therapy and vaccine design.     

Accessory cells in the in vitro generation of type C virus-specific T killer lymphocytes. II. Demonstration of a T helper cell in the spleen of in vivo primed mice

The existence of a helper T cell cooperating with cytolytic T lymphocytes (CTL) in cell-mediated anti-tumor responses specific for the virus-induced FMR antigens can be demonstrated by using unprimed thymocytes as CTL precursors and in vivo primed irradiated spleen cells as helper. The helper T cells express Thy-1.2 and Lyt-1.2 antigens at their surface, but not Lyt-2.2. The helper function required the presence of macrophages to be detected, is antigen specific, and appears unusually radiosensitive compared with previously described helper T cell function.     

The tumoricidal activity of memory CD8+ T cells is hampered by persistent systemic antigen, but full functional capacity is regained in an antigen-free environment

Naive T cells can be tolerized in the periphery by diverse mechanisms. However, the extent to which memory T cells are susceptible to tolerance induction is less well defined. Vaccination of mice with a minimal CTL epitope derived from human adenovirus type 5 E1A in IFA s.c. readily tolerizes naive as well as recently activated CD8(+) T cells due to the overwhelming systemic and persistent presence of the peptide. We have now studied the effect of this peptide on established memory cells, which were induced at least 50 days before by virus vaccination. Memory cells did not undergo peripheral deletion and kept their ability to produce IFN-gamma as well as their cytolytic activity in response to Ag directly ex vivo. However, memory CTL responses in virus vaccinated mice injected with peptide ceased to control tumor outgrowth. Interestingly, functional capacities were regained when T cells were transferred to an Ag-free environment in vivo as determined by their ability to reject an otherwise lethal tumor challenge. Together, these findings indicate that memory CTL responses can be functionally incapacitated, but are not, in contrast to naive or recently activated T cells, irreversibly tolerized by persistent systemic Ag, as memory T cells quickly regain effector function upon disappearance of the Ag.     

A new theory of cytotoxic T-lymphocyte memory: implications for HIV treatment

We use simple mathematical models to examine the dynamics of primary and secondary cytotoxic T-lymphocyte (CTL) responses to viral infections. In particular, we are interested in conditions required to resolve the infection and to protect the host upon secondary challenge. While protection against reinfection is only effective in a restricted set of circumstances, we find that resolution of the primary infection requires persistence of CTL precursors (GTLp), as well as a fast rate of activation of the CTLp. Since these are commonly the defining characteristics of CTL memory, we propose that CTL memory may have evolved in order to clear the virus during primary challenge. We show experimental data from lymphocytic choriomeningitis virus infection in mice, supporting our theory on CTL memory. We adapt our models to HIV and find that immune impairment during the primary phase of the infection may result in the failure to establish CTL memory which in turn leads to viral persistence. Based on our models we suggest conceptual treatment regimes which ensure establishment of CTL memory. This would allow the immune response to control HIV in the long term in the absence of continued therapy.     

Sufficiency of the CD8+ T cell lineage to mount an effective tumoricidal response to syngeneic tumor-bearing novel class I MHC antigens

The origins of "help" in rejection of syngeneic tumors by the CD8 T cell lineage was examined with a model tumor inappropriately expressing novel class I MHC and subject to cytolytic T cell (CTL)-mediated rejection. The requirement for CD4+ Th cells to induce CD8+ CTL effectors in vivo was investigated by using C3H mice selectively depleted of either CD4+ or CD8+ T cells. Rejection of the tumor was vigorous and indistinguishable from normal mice after depletion of CD4+ T cells in vivo. In contrast, in CD8+ T cell-depleted mice tumors grew progressively, confirming that T cells of the CD8+ lineage are required for a tumoricidal immune response, and cells of this lineage are sufficient for a primary response. Taken together, these results demonstrate that, in the absence of CD4+ T cells in vivo, unprimed cells of the CD8+ lineage are fully competent to mount an effective CTL immune response to syngeneic cells expressing novel class I Ag, consistent with the concept that only T cells with class I recognition specificity may be required to satisfy the need for both help and effector functions in the response.     

The BZLF1 homolog of an Epstein-Barr-related gamma-herpesvirus is a frequent target of the CTL response in persistently infected rhesus macaques

Although CD8(+) T lymphocytes targeting lytic infection proteins dominate the immune response to acute and persistent EBV infection, their role in immune control of EBV replication is not known. Rhesus lymphocryptovirus (rhLCV) is a gamma-herpesvirus closely related to EBV, which establishes persistent infection in rhesus macaques. In this study, we investigated cellular immune responses to the rhLCV BZLF1 (rhBZLF1) homolog in a cohort of rhLCV-seropositive rhesus macaques. rhBZLF1-specific IFN-gamma ELISPOT responses ranging between 56 and 3070 spot-forming cells/10(6) PBMC were detected in 36 of 57 (63%) rhesus macaques and were largely mediated by CD8(+) T lymphocytes. The prevalence and magnitude of ELISPOT responses were greater in adult (5-15 years of age) rather than juvenile macaques (<5 years of age), suggesting that rhBZLF1-specific CTL increase over time following early primary infection. A highly immunogenic region in the carboxyl terminus of the rhBZLF1 protein containing overlapping CTL epitopes restricted by Mamu-A*01 and other as yet unidentified MHC class I alleles was identified. The presence of a robust CD8(+) T lymphocyte response targeting this lytic infection protein in both rhesus macaques and humans suggests that these CTL may be important for immune control of EBV-related gamma-herpesvirus infection. These data underscore the utility of the rhLCV-macaque model for studies of EBV pathogenesis.     

Tolerance induction by transcutaneous immunization through ultraviolet-irradiated skin is transferable through CD4+CD25+ T regulatory cells and is dependent on host-derived IL-10

UV radiation of the skin impairs immune responses to haptens and to tumor Ags. Transcutaneous immunization (TCI) is an effective method of inducing immune responses to protein and peptide Ag. We explore the effect of UV irradiation on TCI. The generation of Ag-specific CTL to OVA protein, but not class I MHC-restricted OVA peptide, is inhibited by TCI through UV-irradiated skin. Consequently, the induction of protein contact hypersensitivity and in vivo Ag-specific CTL activity following OVA protein immunization is prevented. Application of haptens to UV-exposed skin induces hapten-specific tolerance. We demonstrate that application of protein or class II MHC-restricted OVA peptide to UV-irradiated skin induces transferable Ag-specific tolerance. This tolerance is mediated by CD4(+)CD25(+) T regulatory (T(reg)) cells. These Ag-specific T(reg) cells inhibit the priming of CTL following protein immunization in the presence of CpG adjuvant. IL-10 deficiency is known to prevent hapten-specific tolerance induction. In this study, we demonstrate, using IL-10-deficient mice and adoptive T cell transfer, that IL-10 is required for the direct inhibition of CTL priming following immunization through UV-irradiated skin. However, IL-10 is not required for the induction of T(reg) cells through UV-irradiated skin as IL-10-deficient T(reg) cells are able to mediate tolerance. Rather, host-derived IL-10 is required for the function of UV-generated T(reg) cells. These experiments indicate that protein and peptide TCI through UV-irradiated skin may be used to induce robust Ag-specific tolerance to neo-Ags and that UV-induced T(reg) cells mediate their effects in part through the modulation of IL-10.     

Presence of effector CD8+ T cells in hepatitis C virus-exposed healthy seronegative donors.

CTL responses against multiple hepatitis C virus (HCV) epitopes were detected in 7 of 29 (24.1%) healthy family members (HFM) persistently exposed to chronically HCV-infected patients (HCV-HFM). These precursor CTL were at very low or undetectable frequencies, as determined by limiting dilution analysis. However, when HCV-specific effector CD8+ T cells, freshly isolated from PBMC of HCV-HFM, were assessed by a sensitive enzyme-linked immunospot assay, their frequencies were severalfold higher than those of precursor CTL. These results indicate that the two assays detect two functionally distinct T cell populations and that the effector cells are not assayed by the 51Cr-release assay. Furthermore, the combination of cell depletion and enzyme-linked immunospot analyses showed that the effector cells were confined into a CD8+ CD45RO+ CD28- population. The persistence of effector CD8+ T cells specific for both the structural and nonstructural viral proteins in uninfected HCV-HFM, suggest that: 1) an immunological memory is established upon a subclinical infection without any evidence of hepatitis, in a large cohort of HCV-exposed individuals; 2) because these cells required neither restimulation nor the addition of particular cytokines in vitro for differentiating in effectors, they should be capable of prompt HCV-specific effector function in vivo, possibly providing antiviral protection; and 3) the maintenance of effector T cell responses may be sustained by persisting low-level stimulation induced by inapparent infections.     

The CD8+ T-cell response to an Epstein-Barr virus-related gammaherpesvirus infecting rhesus macaques provides evidence for immune evasion by the EBNA-1 homologue

Epstein-Barr virus (EBV) infection persists for life in humans, similar to other gammaherpesviruses in the same lymphocryptovirus (LCV) genus that naturally infect Old World nonhuman primates. The specific immune elements required for control of EBV infection and potential immune evasion strategies essential for persistent EBV infection are not well defined. We evaluated the cellular immune response to latent infection proteins in rhesus macaques with naturally and experimentally acquired rhesus LCV (rhLCV) infection. RhLCV EBNA-1 (rhEBNA-1) was the most frequently targeted latent infection protein and induced the most robust responses by peripheral blood mononuclear cells tested ex vivo using the gamma interferon ELISPOT assay. In contrast, although in vitro stimulation and expansion of rhLCV-specific T lymphocytes demonstrated cytotoxic T-lymphocyte (CTL) activity against autologous rhLCV-infected B cells, rhEBNA-1-specific CTL activity could not be detected. rhEBNA-1 CTL epitopes were identified and demonstrated that rhEBNA-1-specific CTL were stimulated and expanded in vitro but did not lyse targets expressing rhEBNA-1. Similarly, rhEBNA-1-specific CTL clones were able to lyse targets pulsed with rhEBNA-1 peptides or expressing rhEBNA-1 deleted for the glycine-alanine repeat (GAR) but not full-length rhEBNA-1 or rhLCV-infected B cells. These studies show that the rhLCV-specific immune response to latent infection proteins is similar to the EBV response in humans, and a potential immune evasion mechanism for EBNA-1 has been conserved in rhLCV. Thus, the rhLCV animal model can be used to analyze the immune responses important for control of persistent LCV infection and the role of the EBNA-1 GAR for immune evasion in vivo.     

Disparate primary and secondary allospecific CD8+ T cell cytolytic effector function in the presence or absence of host CD4+ T cells

The role of CD4+ T cells in promoting CD8+ T cell effector activity in response to transplant Ags in vivo has not been reported. We used a hepatocellular allograft model known to initiate both CD4-dependent and CD4-independent rejection responses to investigate the contribution of CD4+ T cells to the development, function, and persistence of allospecific CD8+ T cell effectors in vivo. Complete MHC-mismatched hepatocellular allografts were transplanted into C57BL/6 (CD4-sufficient) or CD4 knockout (CD4-deficient) hosts. The development of in vivo allospecific cytotoxicity was determined by clearance of CFSE-labeled target cells. CD8+ T cell cytotoxic effector activity was enhanced in response to allogeneic hepatocellular grafts with a greater magnitude of allocytotoxicity and a prolonged persistence of CTL effector activity in CD4-sufficient hosts compared with CD4-deficient hosts. Cytolytic activity was mediated by CD8+ T cells in both recipient groups. In response to a second hepatocyte transplant, rejection kinetics were enhanced in both CD4-sufficient and CD4-deficient hepatocyte recipients. However, only CD4-sufficient hosts developed recall CTL responses with an augmented magnitude and persistence of allocytotoxicity in comparison with primary CTL responses. These studies show important functional differences between alloreactive CD8+ T cell cytolytic effectors that mature in vivo in the presence or absence of CD4+ T cells.     

Induction of AIDS virus-specific CTL activity in fresh, unstimulated peripheral blood lymphocytes from rhesus macaques vaccinated with a DNA prime/modified vaccinia virus Ankara boost regimen

The observed role of CTL in the containment of AIDS virus replication suggests that an effective HIV vaccine will be required to generate strong CTL responses. Because epitope-based vaccines offer several potential advantages for inducing strong, multispecific CTL responses, we tested the ability of an epitope-based DNA prime/modified vaccinia virus Ankara (MVA) boost vaccine to induce CTL responses against a single SIVgag CTL epitope. As assessed using both 51Cr release assays and tetramer staining of in vitro stimulated PBMC, DNA vaccinations administered to the skin with the gene gun induced and progressively increased p11C, C-->M (CTPYDINQM)-specific CD8+ T lymphocyte responses in six of six Mamu-A*01+ rhesus macaques. Tetramer staining of fresh, unstimulated PBMC from two of the DNA-vaccinated animals indicated that as much as 0.4% of all CD3+/CD8alpha+ T lymphocytes were specific for the SIVgag CTL epitope. Administration of MVA expressing the SIVgag CTL epitope further boosted these responses, such that 0.8-20.0% of CD3+/CD8alpha+ T lymphocytes in fresh, unstimulated PBMC were now Ag specific. Enzyme-linked immunospot assays confirmed this high frequency of Ag-specific cells, and intracellular IFN-gamma staining demonstrated that the majority of these cells produced IFN-gamma after peptide stimulation. Moreover, direct ex vivo SIV-specific cytotoxic activity could be detected in PBMC from five of the six DNA/MVA-vaccinated animals, indicating that this epitope-based DNA prime/MVA boost regimen represents a potent method for inducing high levels of functionally active, Ag-specific CD8+ T lymphocytes in non-human primates.     

Testicular immune privilege promotes transplantation tolerance by altering the balance between memory and regulatory T cells

Immune responses are suppressed in immunologically privileged sites, which may provide a unique opportunity to prolong allograft survival. However, it is unknown whether testicular immune privilege promotes transplantation tolerance. Mechanisms underlying immune privilege are also not well understood. Here we found that islet transplantation in the testis, an immunologically privileged site, generates much less memory CD8(+) T cells but induces more Ag-specific CD4(+)CD25(+) regulatory T cells than in a conventional site. These CD4(+)CD25(+) cells exhibited the suppression of alloimmune responses in vivo and in vitro. Despite the immune regulation, intratesticular islet allografts all were rejected within 42 days after transplantation although they survived longer than renal subcapsular islet allografts. However, blocking CD40/CD40L costimulation induced the tolerance of intratesticular, but not renal subcapsular, islet allografts. Tolerance to intratesticular islet allografts spread to skin allografts in the non-privileged sites. Either transfer of memory CD8(+) T cells or deletion of CD25(+) T cells in vivo broke islet allograft tolerance. Thus, transplantation tolerance requires both costimulatory blockade, which suppresses acute allograft rejection, and a favorable balance between memory and regulatory T cells that could favorably prevent late allograft failure. These findings reveal novel mechanisms of immune privilege and provide direct evidence that testicular immune privilege fosters the induction of transplantation tolerance to allografts in both immunologically privileged and non-privileged sites.     

IL-2 during in vitro priming promotes subsequent engraftment and successful adoptive tumor immunotherapy by persistent memory phenotypic CD8(+) T cells

Adoptive T cell tumor immunotherapy potentially consists of two protective components by the transferred effector cells, the immediate immune response and the subsequent development of memory T cells. The extent by which adoptively transferred CD8(+) CTL are destined to become memory T cells is ambiguous as most studies focus on the acute effects on tumor shortly following adoptive transfer. In this study we show that a substantial fraction of the input CTL develop into memory cells that reject a s.c. tumor challenge. The use of exogenous IL-2 or a combination of IL-2 and IL-4, but not solely IL-4, during the ex vivo culture for the CTL inoculation was necessary for efficient development of CD8(+) memory T cells. Thus, an important component of adoptive immunotherapy using CTL is the production of CD8(+) Ag-specific memory cells which is primarily favored by IL-2 receptor signaling during ex vivo generation of the effector CTL.     

Dominant induction of vaccine antigen-specific cytotoxic T lymphocyte responses after simian immunodeficiency virus challenge

Cytotoxic T lymphocyte (CTL) responses are crucial for the control of human and simian immunodeficiency virus (HIV and SIV) replication. A promising AIDS vaccine strategy is to induce CTL memory resulting in more effective CTL responses post-viral exposure compared to those in natural HIV infections. We previously developed a CTL-inducing vaccine and showed SIV control in some vaccinated rhesus macaques. These vaccine-based SIV controllers elicited vaccine antigen-specific CTL responses dominantly in the acute phase post-challenge. Here, we examined CTL responses post-challenge in those vaccinated animals that failed to control SIV replication. Unvaccinated rhesus macaques possessing the major histocompatibility complex class I haplotype 90-088-Ij dominantly elicited SIV non-Gag antigen-specific CTL responses after SIV challenge, while those induced with Gag-specific CTL memory by prophylactic vaccination failed to control SIV replication with dominant Gag-specific CTL responses in the acute phase, indicating dominant induction of vaccine antigen-specific CTL responses post-challenge even in non-controllers. Further analysis suggested that prophylactic vaccination results in dominant induction of vaccine antigen-specific CTL responses post-viral exposure but delays SIV non-vaccine antigen-specific CTL responses. These results imply a significant influence of prophylactic vaccination on CTL immunodominance post-viral exposure, providing insights into antigen design in development of a CTL-inducing AIDS vaccine.     

T lymphocyte cytotoxicity with natural varicella-zoster virus infection and after immunization with live attenuated varicella vaccine

Varicella-zoster virus (VZV) specific cytotoxicity was investigated during acute primary VZV infection, in naturally immune subjects and after vaccination with the live attenuated varicella vaccine by using T cell cultures (TCC) generated by stimulating PBMC with VZV Ag and autologous VZV-superinfected lymphoblastoid cell lines as targets. Lysis of VZV-infected lymphoblastoid cell lines was observed by TCC from acutely infected subjects, naturally immune subjects, and recipients of the varicella vaccine. VZV glycoprotein I induced cytotoxic T cells but killing was less efficient than killing by TCC stimulated with VZV Ag. The TCC were primarily CD4+ (mean 86.6%) T lymphocytes with 15.2% of the cells coexpressing Leu-19. TCC were predominantly restricted by HLA class II as demonstrated by lack of any blocking using class I mAb and blocking of 15 to 71% by L243, a mAb to class II. Unrestricted killing as measured by killing of K562 cells occurred in all TCC but was minimally greater than that observed against uninfected autologous targets. Phenotypes of PBMC during acute infection had an initial increase in CD4+ cells and an overall decrease in the percentage of circulating Leu-11+ (CD16). No enhanced K562 killing was demonstrated in PBMC from subjects with acute infection compared to subjects without infection. CD4+ CTL may function as an important primary host response in acute varicella. Immunization with live attenuated varicella vaccine induced VZV-specific, memory CTL responses comparable to those of naturally immune subjects. The demonstration of their persistence long after primary VZV infection may indicate a role for CTL in restriction of viral replication during episodes of VZV reactivation from latency.