IL-7 and lymphopenia at different stages of HIV-1 infection (I) Since IL-7 has been designated to be a homeostatic factor increasing the

number of T cells during lymphopenic conditions, we sought to investigate its role during different stages of HIV-1 infection which are associated with different CD4+ T cell numbers. Since it is still not fully understood what exactly regulates the levels of IL-7 in HIV-1 infection, we studied if the degree of lymphopenia has a direct impact and can modify the levels of IL-7.

Patients with primary HIV-1 infection show a reduced number of CD4+ T cells that will increase after the infection is somewhat controlled either by the hosts own immune system and/or by treatment with ART. When measuring IL-7 serum levels at baseline we found them to be high in the majority of primary HIV-1 patients investigated, whereas the CD4+ T cell counts were at different levels showing no correlation with IL-7 at this time point. In addition, there was no correlation with CD8+ T cells alone; however the significant association seen with the total levels of CD3+ T cells still reflects an effect of lymphopenia on the increased IL-7 level, as previously seen in chronic infection(Napolitano et al., 2001) (Mastroianni et al., 2001) (Llano et al., 2001) (Fry et al., 2001) (Fig 4a). Also, in patients where the time of ARS was known, the IL-7 levels were decreasing with time, reflecting the recovery of T cell numbers although this correlation was not significant (p=0,07). It was also previously shown that IL-7 levels in primary HIV-1 infection were elevated and correlated with CD4+ T cell reconstitution (Sasson et al., 2006b); treatment with ART resulted in normalized plasma IL-7 level in patients with primary HIV-1 infection.

In our study, the CD4+ T cell counts increased in primary infected patients that had received ART therapy for 5 months (12 out of 14 patients), as predicted.

The IL-7 levels were instead increased during this period and thus did not appear to be regulated by the lymphopenia in this setting. However the baseline IL-7 values correlated with the CD4+ and CD3+ T cell recovery seen after 5 months of therapy (Fig. 4b and c). This possibly indicates that if IL-7 levels are high before the onset of ART therapy, there is a better recovery of the T cells than if the IL-7 levels are low. This observation may suggest that administration of IL-7 in primary infected patients prior to ART may be beneficial for T cell recovery.

Figure 4. Correlations between serum IL-7 and T cell counts during primary HIV-1 infection. A cohort of 14 HIV-1 infected patients was analyzed at acute infection and 5 months later following ART administration. a) Serum IL-7 levels are compared to CD3+ T cell count.

Changes in CD4+ T cell count (b) and CD3+ T cell count (c) during a 5 month period in correlation with levels of IL-7 at baseline are shown.

In chronic HIV infection a correlation has been observed with high IL-7 levels in patients experiencing a diverse treatment history with a CD4+ T cell count less than 200 cells/μl, and lower levels of IL-7 if the CD4+ T cell numbers exceed this amount. In order to exclude the influence of ART on the level of IL-7 we studied treatment naïve patients by measuring the IL-7 levels 2-4 times during a period of 13-56 months. In this setting we found no association between CD4+ T cell count and IL-7, but instead we did see a correlation between CD8+

and CD3+ T cells and IL-7 level indicating a possible role of lymphopenia in serum IL-7 regulation (Fig 5a and b). However, the changes observed in the T cell count and IL-7 levels did not correlate during this period of observation.

a b c

Figure 5. Correlation between T cell counts and IL-7 levels in chronic HIV-1 infection.

Serum IL-7 levels were analyzed 2-4 times during a period of 13-56 months in a group of ART-naïve HIV-1 infected patients. a) serum IL-7 level is compared to CD8+ T cell count. b) serum IL-7 level is compared to CD3+ T cell count.

By investigating several, treatment naïve individuals during the same time period (13-56 months), and relating the changes in IL-7 levels and CD4+ and CD8+ T cell counts, we found a wide variation that appears to reflect individual differences. This suggests that during chronic HIV-1 infection, IL-7 levels only play a role in certain patients, perhaps in an immune system that is not yet exhausted and still can respond to the homeostatic pressure.

We studied IL-7 levels in LTNPs, characterized by CD4+ T cell counts above 500 cells/μl and control of viral replication for 7 to 10 years without ART, and we compared this group of asymptomatic individuals to chronically HIV-1 infected patients on ART. There was no difference in the IL-7 levels between the two groups. In some of the LTNPs investigated eventually the CD4+ T cell counts decline under 500 cells/μl and we made the interesting observation that these individuals showed a higher level of IL-7 before their progression as compared to the LTNPs that kept their high CD4+ T cell count and remained healthy (Fig 6). This finding could perhaps be of use as a prognostic value in the clinic, although that requires a more extensive study to be performed in order to generate a steady cut-off value given that the method of detection is stable. More likely, to follow variations of IL-7 levels in individual patients over time may help to predict the loss of LTNP state. The interesting question in this context is of course why it is not the contrary namely that patients who

a b

progress should have lower IL-7 levels, since the IL-7 is a survival factor for T cells and should keep their number stable. Some other groups have shown on the other hand that IL-7 could increase viral replication from reservoirs and thus increase the viral load (Wang et al., 2005) (Smithgall et al., 1996); thus elevated levels of IL-7 may lead to increased viral replication and loss of CD4+

T cells. It might also be so that the remaining cells somewhat are defective in responding to IL-7, due to down-regulation of the IL-7Rα (paper II), and by that way less IL-7 is used and IL-7 levels accumulate in body fluids. That this may be the case has been recently shown by the work of Sasson and collaborators (Sasson et al., 2006b) showing that changes in CD4+ T cell expression of IL-7R components were evident in patients with LTNP who lost viral control.