The unusually low fractions of HIV-specific CD8+ Tdt and Tcm detected in progressive infection suggest impaired differentiation may be a key factor in HIV-specific CD8+ T cell dysfunction and progression to acquired immune deficiency syndrome (AIDS) [9, 10, 14]. Since CD8+ T cells themselves are rarely HIV-infected, their impairment relates indirectly to HIV replication. Thus, whether HAART normalizes HIV-specific CD8+ T cell differentiation in chronic infection remains unknown. In this study, we used overlapping peptides encompassing all of HIV Gag, Pol, Env and Nef to broadly examine HIV-specific CD8+ T cell responses and address the impact successful antiretroviral suppression of HIV replication had on these responses. We compared evolution of the magnitude, specificity and character of HIV-specific CD8+ T cell responses in two groups of individuals, both chronically infected with HIV, but distinguished by continuously detectable versus undetectable HIV replication over study periods from 2-9 years.
We found that the HIV-specific CD8+CD45RA+ population, which includes Tdt, were relatively scarce throughout the detectable virus replication group and most of the undetectable virus replication group. While such paucity was previously described in studies focused on individual epitopes, the use of genome wide peptide pools herein clarifies its broader relevance to the HIV-specific CD8+ T cell response [9, 14]. The percentage of CD45RA+ effectors, together with the absolute number of HIV-specific CD8+ T cells remained relatively constant in both groups, consistent with establishment of stable ratios under chronic conditions of either antiretroviral suppression or HIV replication. While the overall similar character of HIV-specific CD8+ T cell responses in both groups also supports the notion of intransigence to antiretroviral therapy, we did not compare the same individuals before and after introduction of successful antiretroviral therapy. Therefore, improvements in the character of HIV-specific CD8+ T cell responses occurring early after onset of antiretroviral therapy would not have been detected in this study.
Although these responses would likely not be detectable by flow cytometry with individual peptides, we detected CD8+ HIV-specific IL-2 responses in nearly half of the chronically infected individuals we tested by ELISPOT with peptide sets covering the entire HIV genome. The absolute and relative frequencies of HIV-specific CD8+ T cells producing IL-2 were, nonetheless, very low in both groups, confirming that few, or in some cases, none of the HIV-specific CD8+CD45RA- T cells were Tcm. The low numbers of HIV-specific IL-2 sfc indicate that only a very small fraction of the HIV-specific CD8+CD45RA- IFN-γ producing T cells are Tcm. Therefore, prolonged or repeated expansion of HIV-specific effector cells skews the HIV-specific CD8+ T cell population towards a predominant effector memory (Tem) phenotype and this skewing is not reversed by long-term suppression of HIV replication.
Identifying HIV-specific CD8+ Tcm other than by expression of surface markers such as CCR7 could conceivably underestimate Tcm numbers, but proliferative function as indicated by IL-2 production may be a more meaningful discriminator of HIV-specific CD8+ Tcm. The lower fraction of HIV-specific CD8+ Tcm thus identified in the group with detectable HIV replication actually reflected higher absolute Tem numbers in this group, not fewer Tcm. Absolute Tcm numbers changed little over time in either group, implying a rough equilibrium of HIV-specific T cell subset distribution under relatively stable conditions of antiretroviral suppression or HIV replication. Thus, it appears that in stable settings of either viral replication or antiretroviral suppression, individuals reach a steady-state HIV-specific CD8+ T cell response, analogous and related to virus load set-point . This equilibrium would shift with acute expansion of Tem upon de novo viral replication or contraction of Tem with introduction of effective HAART. In our study, where initial samples were drawn at least 3 months after establishment of viral suppression or viral replication, the absolute magnitude, general specificity and subset distribution of the CD8+ anti-HIV T cell response remained remarkably constant. This suggests an enormous reserve of CD8+ HIV-specific effector cells, although previous or partially effective treatment probably also played a role in the durability of responses in our study group. In progressive infection, HIV-specific CD8+ T cell activation, subset distribution and elimination maintains a set point that slowly decays towards true numerical deficits only in advanced infection. Set point maintenance in treated non-progressing subjects with undetectable viral loads indicates occult viral replication sufficient for continuous recruitment of relatively short-lived CD8+ HIV-specific Tem or some residual conditioning of the immune system that perpetuates aberrant differentiation and subset distribution of HIV-specific CD8+ T cells. Such low level replication, as was recently confirmed in both elite controllers and treated individuals with clinically undetectable virus loads, may be an important component of immune deficits that persist in chronic, treated HIV infection [19, 23].
In humans, there are clear associations between initial suppression of viral replication and emergence of HIV-specific CD8+ T cells as well as between advanced disease progression and decline of HIV-specific CD8+ T cells [24–26]. In macaques, depletion of CD8+ T cells in macaque models of HIV infection directly demonstrated the role that CD8+ T cells play in lowering virus load in acute infection and delaying disease progression in chronic infection . However, the coexistence of strong anti-HIV CD8+ T cell responses with substantial levels of HIV replication that we and others have observed raises questions as to the long term contribution of these natural responses . Some data indicate that clonal restriction of the HIV-specific CD8+ T cell response is the earliest predictor of rapid disease progression, suggesting that the initial dynamics of CD8+ T cell-mediated suppression of viral replication versus viral dissemination, diversification and immune destruction play a major role in determining the steady-state parameters of chronic infection [17, 29]. Our data indicate that antiretroviral treatment in chronic HIV infection shifts the steady state towards slower decay and lower HIV-specific CD8+ Tem frequencies without affecting the overall character of the HIV-specific CD8+ T cell response. Without supplementary immunological conditioning to enhance the efficacy of HIV-specific immunity, the system will return to its original trajectory upon treatment failure or withdrawal.
In summary, we found no evidence that effective antiretroviral therapy influences any feature other than magnitude of HIV-specific CD8+ T cell responses. Low frequencies of CD8+ HIV-specific Tcm and low fractions of CD45RA+ Tdt persisted over prolonged periods of effective HAART, suggesting that abnormal characteristics of the HIV-specific CD8+ T cell response imprinted following acute infection endure immune restoration associated with HAART. However, none of the subjects in our study received antiretroviral therapy during acute infection, which may be more likely to influence HIV-specific CD8+ T cell differentiation and subset distribution than treatment during chronic infection. Novel strategies addressing HIV-specific CD8+ T cell response character will be required to achieve true immune restoration in chronic HIV infection.