Regulatory T cells, immune activation and apoptosis are involved in persistently reduced CD4 counts in HIV-infected immunological non-responders

CD4 counts are persistently reduced and CD4 T lymphocytes are functionally-defective in a percentage of HIV-infected, HAART-treated patients (immunological non-responders –INR-). T regulatory cells (Treg), immuneactivation and apoptosis are suggested to play a role in such CD4 count defects. The PD-1/PD-L1 and the Fas/FasL pathways elicit apoptosis of antigen-specific cells; Treg down-regulate T cell function and induce apoptosis by PD1/PD-L1 interaction. To verify possible role for these mechanisms in INR we enrolled 38 HIV-infected patients with comparable CD4 nadirs and HIV RNA <50 copies/ml who had undergone HAART for >1 year. CD4 counts were >500 cells/l in 15 patients (full responders –FR-); in 23 other patients (INR) CD4 were <200 cells/l. PBMC were stimulated with gag+env or with CMV peptides. Flow-cytometry was used to analyze intra and extracellular PD1 in CD4+ T lymphocytes as well as the expression of PD-L1, TLR2 and TLR4, and of Fas and FasL on immune cells. Treg lymphocytes (CD4+/CD25high/Foxp3+), IL-10 production, activation of caspases 8 and 9, and plasmatic LPS concentration were also examined in all patients. Results showed that in INR compared to FR patients: 1) intra-and extracellular PD1 positive CD4+ T lymphocytes, 2) Fas- and FasL-expressing CD4+T lymphocytes, 3) Treg and IL-10 production, and 4) the percentage of apoptotic cells are increased. Additionally, in INR patients CD14+/TLR2+ TLR4+ cells are augmented, plasmatic LPS concentration is higher, and CD4+ T cells are hyperactivated. These increases are seen both in gag+env- and CMV-stimulated PBMC. Notably, PD-L1 expression was comparable in CD14+ and CD19+ cells of both groups of patients. The augmented plasmatic LPS concentration detected in INR could explain immune activation and presence of higher Treg cell function. PD1/PD-L1interaction results in an increased apoptosis of antigen-specific CD4+ T lymphocytes. IL-10 is also increased in INR, this observation could explain the functional T helper impairments also seen in these patients. Apoptosis plays an important role in the defective immune reconstitution seen in INR.