We found that elite controllers maintained lower levels of activated HIV-specific CD8+ and recently divided HIV-specific CD4+ T cells compared to non-controllers, and this was not explained by increased proportions of Treg cells in elite controllers. Decreased antigen-specific T cell activation was limited to the HIV-specific subsets of cells and was not found in the respective CMV-specific populations. Additionally, the elite controllers possessed the strongest and broadest HIV-specific immune responses, with seven cytokines and chemokines induced by HIV stimulation (IL-2, IFNγ, TNFα, GM-CSF, IP-10, MCP-3 and MIP-1β). In summary, elite controllers maintained the strongest HIV-specific immune responses, with markers of inflammation on HIV-specific cells significantly lower than those in subjects unable to control viral replication.
Elite controllers had the ability to mount a strong and broad antiviral cytokine response when stimulated with a p55 peptide pool, producing IL-2, IFNγ, TNFα, IP-10, GM-CSF, MIP-1β and MCP-3. Whilst the non-controllers also induced some of the same cytokines as elite controllers (IL-2, IFNγ, TNFα, GM-CSF, IP-10 and MCP-3), they produced significantly less of each than elite controllers. The lower quantities of antiviral cytokines may reflect weak antiviral T cell responses and be associated with exhaustion of adaptive immune responses. Antiretroviral therapy was clearly associated with a reduced ability to secrete a broad cytokine and chemokine response, with these individuals only secreting significant amounts of IL-2, IFNγ and IP-10. Interestingly, HAART suppression was associated with the production of IL-13, a cytokine involved in B cell growth and differentiation that can inhibit macrophage inflammatory cytokine production. The reduction of inflammatory cytokines could also be responsible for the reduction in T cell activation and proliferation seen during HAART treatment. Our multiplex cytokine testing data support previous reports of viral control being associated with polyfunctional T cell responses [42
], and the cytokines identified above further clarify which specific responses are associated with control of viral replication during chronic HIV infection.
The increase in the number of Treg cells following HAART that we observed is consistent with some previous studies. Weiss et al.
found an expanded number of Treg cells in HIV+
individuals receiving HAART, with a Treg cell phenotype similar to that of normal donors and cancer patients [26
]. Lim et al.
also observed an increase in the number of Treg cells identified by an increase in FoxP3 mRNA expression in individuals who suppressed viremia with HAART [47
]. Kolte et al.
found that both absolute Treg cell numbers and the percentage of Treg cells were increased after one and five years of receiving HAART and were associated with an increase in the thymic output of naïve Treg cells [48
]. Two other studies showed no effect of HAART on Treg cell numbers despite suppression of viral replication and immunological recovery [49
]. The precise mechanism of Treg cell expansion during HAART remains unknown and requires further investigation. An increase in the peripheral Treg cell pool by proliferation, increased survival of Treg cells or an increase in the thymic generation of Treg cells all could be responsible [41
]. As we saw no correlation between the number of Treg cells and HIV-specific or CMV-specific T cell responses (data not shown), it would appear that Treg cells do not strongly interfere with HIV-specific immune responses, raising the possibility of inducing these cells to ameliorate the effects of immune activation in the setting of high viral loads during chronic HIV infection.
Whilst our data mostly agree with those of Chase et al.
], we did see a difference in which HIV infected group had the highest number of Treg cells. Elite controllers in the Chase et al.
study had the highest number of Treg cells, whereas we saw the highest number of Treg cells in our HAART suppressed group. One possible explanation for this is confounding by age, since older individuals have higher Treg cell numbers [55
]. In both our study and the Chase study the groups with the highest number of Treg cells were also the oldest. In the Chase study elite controllers were the oldest (median age = 54 years), while their HAART suppressed group was the youngest (median age = 46 years). In contrast, our HAART suppressed group was the oldest (median age = 53 years) and the elite controllers were younger (median age = 48 years). Multivariate analysis of our data, which controlled for confounding by age, showed that the increase in Treg cells was due to the therapy and not age. Whether this would be the case in the Chase et al.
study was not addressed [54
In conclusion, lower levels of HIV-specific T cell activation and in vivo
proliferation combined with stronger, broader HIV-specific cytokine responses likely play a role in the control of HIV infection by elite controllers. However, elite controllers do not completely clear the virus [2
] and may eventually lose their elite status and progress towards the development of AIDS [1
]. A therapeutic vaccine or immune modulation that could reduce immune activation, potentially by the induction of Treg cells, and generate a more appropriate balance of immune responses (such as those seen in elite controllers) may allow non-controllers to decrease HIV replication and delay the progression to AIDS.