Altered IL-7 signaling in CD4⁺ T cells from patients with visceral leishmaniasis

Background CD4(+) T cells play a central role in control of L. donovani infection, through IFN-gamma production required for activation of macrophages and killing of intracellular parasites. Impaired control of parasites can in part be explained by hampered CD4(+) T cells effector functions in visceral leishmaniasis (VL) patients. In a recent studies that defined transcriptional signatures for CD4(+) T cells from active VL patients, we found that expression of the IL-7 receptor alpha chain (IL-7R alpha; CD127) was downregulated, compared to CD4(+) T cells from endemic controls (ECs). Since IL-7 signaling is critical for the survival and homeostatic maintenance of CD4(+) T cells, we investigated this signaling pathway in VL patients, relative to ECs. Methods CD4(+) T cells were enriched from peripheral blood collected from VL patients and EC subjects and expression of IL7 and IL7RA mRNA was measured by real time qPCR. IL-7 signaling potential and surface expression of CD127 and CD132 on CD4(+) T cell was analyzed by multicolor flow cytometry. Plasma levels of soluble IL-7 and sIL-7R alpha were measured by ELISA. Result Transcriptional profiling data sets generated previously from our group showed lower IL7RA mRNA expression in VL CD4(+) T cells as compared to EC. A significant reduction was, however not seen when assessing IL7RA mRNA by RT-qPCR. Yet, the levels of soluble IL-7R alpha (sIL-7R alpha) were reduced in plasma of VL patients compared to ECs. Furthermore, the levels of soluble IL-7 were higher in plasma from VL patients compared to ECs. Interestingly, expression of the IL-7R alpha protein was higher on VL patient CD4(+) T cells as compared to EC, with activated CD38(+) CD4(+) T cells showing higher surface expression of IL-7R alpha compared to CD38(-) CD4(+) T cells in VL patients. CD4(+) T cells from VL patients had higher signaling potential baseline and after stimulation with recombinant human IL-7 (rhIL-7) compared to EC, as measured by phosphorylation of STAT5 (pSTAT5). Interestingly, it was the CD38 negative cells that had the highest level of pSTAT5 in VL patient CD4(+) T cells after IL-7 stimulation. Thus, despite unaltered or potentially lowered IL7RA mRNA expression by CD4(+) T cells from VL patients, the surface expression of the IL-7R alpha was higher compared to EC and increased pSTAT5 was seen following exposure to rhIL-7. Accordingly, IL-7 signaling appears to be functional and even enhanced in VL CD4(+) T cells and cannot explain the impaired effector function of VL CD4(+) T cells. The enhanced plasma IL-7 may serve as part of homeostatic feedback mechanism regulating IL7RA expression in CD4(+) T cells.