Person:

Alter, Galit

Background: Virus-specific CD8+ T lymphocytes play a key role in the initial reduction of peak viremia during acute viral infections, but display signs of increasing dysfunction and exhaustion under conditions of chronic antigen persistence. It has been suggested that virus-specific CD8+ T cells with a “polyfunctional” profile, defined by the capacity to secrete multiple cytokines or chemokines, are most competent in controlling viral replication in chronic HIV-1 infection. We used HIV-1 infection as a model of chronic persistent viral infection to investigate the process of exhaustion and dysfunction of virus-specific CD8+ T cell responses on the single-epitope level over time, starting in primary HIV-1 infection. Methods and Findings: We longitudinally analyzed the polyfunctional epitope-specific CD8+ T cell responses of 18 patients during primary HIV-1 infection before and after therapy initiation or sequence variation in the targeted epitope. Epitope-specific CD8+ T cells responded with multiple effector functions to antigenic stimulation during primary HIV-1 infection, but lost their polyfunctional capacity in response to antigen and up-regulated programmed death 1 (PD-1) expression with persistent viremic infection. This exhausted phenotype significantly decreased upon removal of stimulation by antigen, either in response to antiretroviral therapy or by reduction of epitope-specific antigen load in the presence of ongoing viral replication, as a consequence of in vivo selection of cytotoxic T lymphocyte escape mutations in the respective epitopes. Monofunctionality increased in CD8+ T cell responses directed against conserved epitopes from 49% (95% confidence interval 27%–72%) to 76% (56%–95%) (standard deviation [SD] of the effect size 0.71), while monofunctionality remained stable or slightly decreased for responses directed against escaped epitopes from 61% (47%–75%) to 56% (42%–70%) (SD of the effect size 0.18) (p < 0.05). Conclusion: These data suggest that persistence of antigen can be the cause, rather than the consequence, of the functional impairment of virus-specific T cell responses observed during chronic HIV-1 infection, and underscore the importance of evaluating autologous viral sequences in studies aimed at investigating the relationship between virus-specific immunity and associated pathogenesis.

Background: Very little is known about the immunodominance patterns of HIV-1-specific T cell responses during primary HIV-1 infection and the reasons for human lymphocyte antigen (HLA) modulation of disease progression. Methods and Findings: In a cohort of 104 individuals with primary HIV-1 infection, we demonstrate that a subset of CD8+ T cell epitopes within HIV-1 are consistently targeted early after infection, while other epitopes subsequently targeted through the same HLA class I alleles are rarely recognized. Certain HLA alleles consistently contributed more than others to the total virus-specific CD8+ T cell response during primary infection, and also reduced the absolute magnitude of responses restricted by other alleles if coexpressed in the same individual, consistent with immunodomination. Furthermore, individual HLA class I alleles that have been associated with slower HIV-1 disease progression contributed strongly to the total HIV-1-specific CD8+ T cell response during primary infection. Conclusions: These data demonstrate consistent immunodominance patterns of HIV-1-specific CD8+ T cell responses during primary infection and provide a mechanistic explanation for the protective effect of specific HLA class I alleles on HIV-1 disease progression.

Background: Natural killer (NK) cells play a critical role in the host defense against viral infections. However, chronic HIV-1 infection is associated with an accumulation of dysfunctional NK cells that poorly control viral replication. The underlying mechanism for this NK cell mediated dysfunction is not well understood. Certain tumors directly evade NK cell mediated detection by dampening NK cell activity via the downregulation of NKG2D, a potent activating NK cell receptor, via the release of soluble NKG2D-ligands. This results in a potent suppression of NK cell function. We hypothesized that NKG2D-mediated cytotoxicity in chronic HIV-1 infection is impaired and associated with shedding of NKG2D ligand MICA (sMICA). Methods: Flow cytometric analysis was performed on 100 subjects to quantify NKG2D expression and NK cell functional responses (cell activation (CD69) and degranulation (CD107a)) following stimulation with K562 (MHCdevoid, NKG2D target) and 221 cells (MHC devoid, non-NKG2D target). Additionally, the level of MICA and matrix metalloproteases (MMP, involved in NKG2D-ligand shedding) were quantified by quantitative PCR. Finally, the level of secreted MICA was measured by ELISA. Results: Here we show for the first time that chronic HIV-1 infection is associated with a specific defect in NKG2D-mediated activation of NK cells, due to alterations in both the expression and transcription of NKG2D in NK cells. Reduced NKG2D expression was associated with elevated levels of the NKG2D-ligand, MICA, in patient sera, likely released by HIV infected CD4+ T cells due to enhanced enzymatic cleavage, by MMPs, of this stress inducible molecule from the surface of the target cell. Conclusion: Thus, HIV is able to indirectly suppress NK cell mediated functional recognition of HIV-infected CD4+ T cells by enhancing NKG2D-ligand secretion into the peripheral circulation resulting in a profound impairment of NK cell function.