Cytotoxic T cells and human herpes virus infections

Viral persistence alters CD8 T-cell immunodominance and tissue distribution and results in distinct stages of functional impairment

Chronic viral infections often result in ineffective CD8 T-cell responses due to functional exhaustion or physical deletion of virus-specific T cells. However, how persisting virus impacts various CD8 T-cell effector functions and influences other aspects of CD8 T-cell dynamics, such as immunodominance and tissue distribution, remains largely unknown. Using different strains of lymphocytic choriomeningitis virus (LCMV), we compared responses to the same CD8 T-cell epitopes during acute or chronic infection. Persistent infection led to a disruption of the normal immunodominance hierarchy of CD8 T-cell responses seen following acute infection and dramatically altered the tissue distribution of LCMV-specific CD8 T cells in lymphoid and nonlymphoid tissues. Most importantly, CD8 T-cell functional impairment occurred in a hierarchical fashion in chronically infected mice. Production of interleukin 2 and the ability to lyse target cells in vitro were the first functions compromised, followed by the ability to make tumor necrosis factor alpha, while gamma interferon production was most resistant to functional exhaustion. Antigen appeared to be the driving force for this loss of function, since a strong correlation existed between the viral load and the level of exhaustion. Further, epitopes presented at higher levels in vivo resulted in physical deletion, while those presented at lower levels induced functional exhaustion. A model is proposed in which antigen levels drive the hierarchical loss of different CD8 T-cell effector functions during chronic infection, leading to distinct stages of functional impairment and eventually to physical deletion of virus-specific T cells. These results have implications for the study of human chronic infections, where similar T-cell deletion and functional dysregulation has been observed.

CD4(+) T cell-mediated control of a gamma-herpesvirus in B cell-deficient mice is mediated by IFN-gamma

The lack of B cells and antibody does not prevent mice from dealing effectively with a pathogenic gamma-herpesvirus. Both CD4(+) and CD8(+) T cells contribute to the control of virus replication in the respiratory tract, with the depletion of either lymphocyte subset leading to increased titers in the lung. However, the further neutralization of IFN-gamma diminishes the effectiveness of the CD4(+) T cell response and causes substantially increased mortality. Experiments with bone marrow radiation chimeras indicate that the immune CD4(+) effectors operate optimally when there is the potential for direct interaction with virus-infected targets expressing MHC class II glycoproteins, suggesting that the IFN-gamma produced by these lymphocytes is functioning at short range. The numbers of latently infected cells in the spleens of carrier mice are also significantly increased by the concurrent depletion of both the CD4(+) population and IFN-gamma. These experiments raise the possibility that the defective control of intercurrent gamma-herpesvirus infections in patients with AIDS not only is due solely to the absence of helper T cells but also reflects the loss of an important set of CD4(+) effectors.

Differential effect of phototherapy on the activities of human natural killer cells and cytotoxic T cells

Exposure to ultraviolet B (UV-B) light is recognized to induce suppression of certain immune responses, particularly delayed hypersensitivity. However, its effect on cytotoxic T lymphocyte (CTL) activity, of major importance in the resistance to viruses and tumours, has not been assessed to the same extent. In this study five normal subjects, seropositive for herpes simplex virus (HSV), underwent a standard course of broadband UV-B therapy, as used in the treatment of psoriasis. They received whole-body irradiation thrice weekly for four weeks with incremental doses dependent on skin type. Blood samples were taken immediately before, at two time points during, and at the end of the therapy. An HSV-specific CTL assay was performed using autologous B cells transformed with Epstein-Barr virus as targets. No consistent modulation in CTL activity was obtained as a result of the therapy. The CTLs were separated into CD4 and CD8 subsets by positive selection and, again, no effect of irradiation on CTL activity within each of these two populations was observed. In contrast, the natural killer (NK) cell activity, assessed by the lysis of K562 cells, was significantly reduced at the first time point after the initiation of the phototherapy in all five subjects, and it continued to decline as the treatment progressed. Thus a differential effect of UV-B exposure on cytotoxic activity has been demonstrated: the HSV-specific CTL response is unchanged, while the NK response is suppressed.

Kinetic analysis of the specific host response to a murine gammaherpesvirus

Respiratory infection of BALB/c mice with the murine gammaherpesvirus 68 (MHV-68) induces the clonal expansion of virus-specific cytotoxic T-lymphocyte (CTL) precursors (CTLp) in the regional, mediastinal lymph nodes (MLN). Some of these CTLps differentiate to become fully functional CTL effectors, which can be detected in both the lymphoid tissue and in the site of pathology in the lung. Though the lymph nodes and spleen harbor substantial populations of latently infected B cells for life, the level of virus-specific CTL activity decreases rapidly in all sites. The CD8+ CTLp numbers fall to background levels in the MLN within several months of the termination of the productive phase of MHV-68 infection in the respiratory epithelium but are maintained at relatively low frequency in the spleen. The continued presence of a gamma interferon-producing, MHV-68-specific CD4+ set can also be demonstrated in cultured spleen cells. The virus-specific immunoglobulin G (IgG) response is slow to develop, with serum neutralizing antibody and enzyme-linked immunosorbent assay titers continuing to rise for several months. The level of total serum IgG increases dramatically within 2 weeks of infection, probably as a consequence of polyclonal B-cell activation, and remains high. The immune response profile is clearly influenced by the persistence of this DNA virus.

Progressive loss of CD8+ T cell-mediated control of a gamma-herpesvirus in the absence of CD4+ T cells

A unique experimental model has been developed for dissecting the integrity of CD8+ T cell-mediated immunity to a persistent gammaherpesvirus under conditions of CD4+ T cell deficiency. Respiratory challenge of major histocompatibility complex class II -/- and +/+ C57BL/6J mice with the murine gammaherpesvirus 68 (MHV-68) leads to productive infection of both lung and adrenal epithelial cells. Virus titers peak within 5-10 d, and are no longer detected after day 15. Persistent, latent infection is established concurrently in splenic and lymph node B cells, with higher numbers of MHV-68+ lymphocytes being found in all lymphoid sites analyzed from the +/+ mice concurrent with the massive, but transient splenomegaly that occurred only in this group. From day 17, however, the numbers of infected B lymphocytes were consistently higher in the -/- group, while the frequency of this population diminished progressively in the +/+ controls. Infectious MHV-68 was again detected in the respiratory tract and the adrenals of the -/- (but not the +/+) mice from day 22 after infection. The titers in these sites rose progressively, with the majority of the -/- mice dying between days 120 and 133. Even so, some CD8+ effectors were still functioning as late as 100 d after infection. Depletion of CD8+ T cells at this stage led to higher virus titers in the -/- lung, and to the development of wasting in some of the -/- mice. Elimination of the CD8+ T cells from the +/+ group (day 80) increased the numbers of MHV-68+ cells in the spleen, but did not reactivate the infection in the respiratory tract. The results are consistent with the interpretation that CD8+ T cell-mediated control of this persistent gammaherpesvirus is progressively lost in the absence of the CD4+ T cell subset. This parallels what may be happening in AIDS patients who develop Kaposi's sarcoma and various Epstein Barr virus associated disease processes.

Identification of epitope mimics recognized by CTL reactive to the melanoma/melanocyte-derived peptide MART-1(27-35)

CTL reactivity to the epitope MART-1(27-35), of the melanoma (self) antigen MART-1/melan A is frequently observed in tumor-infiltrating lymphocytes and may be readily elicited from the peripheral blood of melanoma patients that express HLA-A*0201. Available data suggest that these observations contrast with those made for other HLA-A*0201-presented melanoma self antigens regarding the regularity of observed CTL responses. Based on preliminary findings, we hypothesized that the CTL response to MART-1 might be augmented in part by T cell encounters with peptides derived from sources other than MART-1, which show sequence similarity to MART-1(27-35). To test this idea, a protein database search for potential MART-1 epitope mimics was done using criteria developed from analyses of effector recognition of singly-substituted peptide analogues of MART-1(27-35). Synthetic peptides were made for a portion of the sequences retrieved; 12/40 peptides tested were able to sensitize target cells for lysis by one or more anti-MART-1 effectors. The peptides recognized correspond to sequences occurring in a variety of proteins of viral, bacterial, and human (self) origin. One peptide derives from glycoprotein C of the common pathogen HSV-1; cells infected with recombinant vaccinia virus encoding native glycoprotein C were lysed by anti-MART-1 effectors. Our results overall indicate that sequences conforming to the A2.1 binding motif and possessing features essential to recognition by anti-MART-1 CTL occur frequently in proteins. These findings further suggest that T cells might encounter a variety of such sequences in vivo, and that epitope mimicry may play a role in modulating the CTL response to MART-1(27-35).

Human monoclonal antibodies to cytomegalovirus. Characterization and recombinant expression of a glycoprotein-B-specific antibody

Human monoclonal antibodies (mAb) to human cytomegalovirus (HCMV) were established from spleen cells of a HCMV-positive donor. The antibodies (gamma 3, lambda) secreted from a stable heterohybridoma cell line were further characterized by immunoprecipitation and immune-fluorescence microscopy using HCMV infected cells and recombinant cell lines expressing HCMV glycoprotein B. The antibody reacted with the entire glycoprotein B or the extracellular domain expressed as glycoprotein-B--beta-galactosidase fusion protein in the native state, but the antibody was not neutralizing HCMV. Denatured and reduced forms of glycoprotein B were not recognized by this antibody, however, native glycoprotein B on the surface of infected cells was detected efficiently. The genes encoding the Fab part of the antibody were cloned and expressed in Escherichia coli. Recombinant Fab fragments specifically binding the extracellular domain of glycoprotein B could easily be isolated from the periplasmic space. Recombinant antibodies provide the opportunity to modify effector functions and to add tags to diagnostic antibodies for more efficient detection of CMV-infected cells.