Understanding over time changes in T cell populations in HIV infected and HCV exposed patients help to get a better knowledge about the impact of prolonged ART and HCV elimination with DAAs on HIV reservoir size dynamic and immune senescence 15.
Overall, we observed a general decline in cell senescence since the proportion of CD8+ TN, CD8+ TCM, CD8+ TEM (CD28+) increased over time and the proportion of CD8+ TTM, CD8+ TEM (CD28-) decreased. The effect of ART regulation on early immune senescence has already been described in patients with chronic HIV infection and correlates with cessation of antigenic stimulation of HIV-specific CD8+ lymphocytes 27, 28, 29. However, to our knowledge, this is the first study that identifies the same phenomenon in HIV infected patients previously exposed to HCV (both chronically coinfected and spontaneous clearers), which could possibly be explained by the end of the antigenic stimulation and sustained HCV-specific CD8+ cell death.
On the other hand, the most senescent population considered in this study, CD8+ TEMRA (CD28-CD57+), increased its proportion at the end of the study for HIV+ and HIV+/HCV+ patients, and it was only reduced for HIV+/HCV- individuals. During natural HIV infection, immune senescence increases with the accumulation of highly differentiated CD4+ and CD8+ cells that have lost the expression of CD28 and gained that of CD57 (CD8+ TEMRA). This may cause premature immune aging and contributes to the onset of acquired immune deficiency syndrome (AIDS) or serious non-AIDS-related pathologies 30, in the absence of ART.
Despite successful ART, HIV infected individuals fail to normalize CD8+ T cell activation (HLADR+CD38+), which leads to a chronic inflammatory state and the appearance of comorbidities 31, 32, 33. This could be reflected in the failure to normalize the proportion of late-senescent cells despite the control of viral replication as was observed in both HIV+ and HIV+/HCV+ patients, who still show altered levels of pro-inflammatory cytokines even after the elimination of HCV with DAAs 34, 35, 36, 37. However, the significant increase observed in CD28+ TEM in all the studied groups, and more importantly in senescent CD57+ TEMRA, may reflect an antigenic stimulation of the cytotoxic T cells, in both HIV+ and HIV+/HCV+ patients differing of the spontaneous clearers group that showed a mild decrease in TEMRA. HIV+/HCV- individuals could managed to control immune aging as the potent immune response that allowed them to spontaneously clarify HCV infection.
The presence of HCV-specific CD8+ T cells, previously identified, with phenotypic features of T-cell exhaustion and memory, both before and after treatment with direct acting antiviral (DAA) agents may contribute to the differences observed in the spontaneous clearers group 38.
Regarding CD4+ T cells, a similar dynamic of CD28 expressing cells was observed but only in non-effector CD27+ cells (CD4+ TN, CD4+TCM, CD4+ TTM). An increased number of CD28 expressing cells in HIV-infected CD4+ cells has been related to a high intracellular concentration of viral accessory proteins Nef and Vpu 39. In fact, our group previously observed a generalized increase over time of single and multiple spliced viral RNA transcripts in the same cohort of patients (unpublished data). However, this would not explain why the dynamic of CD28 seemed to be CD27-dependent on CD4+ cells. This result may be explained by the fact that CD27 promotes T cell survival 40, 41, 42. This could in turn promote resistance to apoptosis of infected CD4+ cells, viral transcription and the effect of Nef and Vpu on CD28.
As discussed above, effector CD4+ cells (CD27-) do not show the same dynamic as that observed in CD8+ cells. CD4+ TEM TH1 cells are especially noteworthy, as they show an identical dynamic to that of the HIV reservoir size in resting (r)CD4+ T cells (CD4+CD25-CD69-HLA-DR-) in the same patient cohort 15. Both viral reservoir size and proportion of CD4+ TEM TH1 significantly decreased in HIV+/HCV- patients and seemed to remain stable in HIV+ and HIV+/HCV+ individuals. Previously, it has been described that CD4+ TCM and TTM are major viral compartments 43 and that under ART, long-lived CD4+ TEM cells replenish the majority of the stable HIV reservoir 44. Moreover, there is now increasing evidence that another cellular subtype, TFH cells both lymph node resident and circulating, would also comprise a large part of the viral reservoir 45, 46. We can relate TEM TH1 and TFH cells since most TFH present a TH1 phenotype (TFH1) with an enhanced production of IFN-γ 47. Thus, we cannot rule out the possibility that the significant decrease in CD4+ cells with TH1 phenotype in HIV+/HCV- individuals could be related to a decrease in TEM TH1 and/or TFH1 cells. Since only TFH1 cells can express the CD4+CD25-CD69-HLA-DR- phenotype 19, 48, 49, 50, 51, these cells correlate to the reduced viral reservoir size in rCD4+ T cell of spontaneous clarifiers 15.
During HIV infection, immune activation and the production of proinflammatory cytokines such as IFN-γ, IL-21, IL-6 and IP-10 increase. IFN-γ is the main stimulator of differentiation from TN to TH1 cells and from TFH0 to TFH1 cells 47. Moreover, HIV+/HCV- patients may initially have a strong CD4+ TH1 response, as it has been related to spontaneous clearance of HCV 52, 53. This could explain, in the context of simultaneous coinfection, the large size of the HIV reservoir observed in these patients prior to follow-up 13. However, over time, we observed that this response stabilized and reservoir diminished. We hypothesized that the decrease of the HIV reservoir size in HIV+/HCV- individuals was related to the decrease in CD4+ latently infected cells with TH1 phenotype since the apparent control of infection by these individuals would reduce immune activation and the production of proinflammatory cytokines. With the reduction of IFN-γ in particular, the proportion of TFH1 cells would decline and with them the amount of proviral DNA copies integrated by 106 CD4+CD25-CD69-HLA-DR- cells.
A decrease in the proportion of TH1 cells and consequently in IFN-γ production would be expected to increase the proportion of TH2 46. However, our results showed a decrease in the proportion of these cells for both HIV+ and HIV+/HCV- patients. Other authors have previously described that the dramatically increased humoral TH2 response during HIV infection stabilizes under ART 54. Therefore, we hypothesized that the generalized decrease in TH2 cell prevalence may be due to a normalization of the TH2 response, and that changes in the HIV viral reservoir are better explained by the dynamic of cellular populations of TH1 phenotype. Further research should be undertaken to investigate TH1 and TH2 cytokine production in these patients.
A decrease in antigen presentation of HIV-specific CD8+ lymphocytes occurs not only in HIV-infected patients under ART but also in patients chronically infected or that had been infected and recovered with HCV.
However, despite successful ART, only HCV spontaneous clarifiers seem to normalize late senescent cells. Furthermore, these patients seem to better control proinflammatory responses and the production of cytokines such as IFN-γ, which in turn would be related with a decrease in the number of TH1 cells, which harbor proviral DNA.