Interleukin-3 and granulocyte-macrophage colony-stimulating factor expression, a biomarker of memory CD8⁺ T cell immunity and vaccine efficacy

Interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) exhibit overlapping activities and are produced by activated T cells. In this study, the role of IL-3 and GM-CSF in CD8⁺ T cells were assessed following HIV-1 prime-boost immunization. Data indicate that the expression of IL-3/GM-CSF by HIV-specific effector CD8⁺ T cells is vaccine delivery route and time dependent, where purely systemic, intramuscular i.m./i.m. vaccination induce elevated levels of IL-3 in HIV-specific CD8⁺ T cells compared to purely mucosal, intranasal i.n./i.n. immunization. Interestingly, the GM-CSF expression was optimal only following i.n./i.m. delivery. Data also revealed that peak IL-3 mRNA and protein expression in CD8⁺ T cells were detected 16-20 h of K^dGag_197-205 peptide stimulation, whereas the expression kinetics of GM-CSF was similar to IFN-γ. Next the IL-3 and GM-CSF expression in HIV-specific CD8⁺ T cells were assessed at acute, effector and memory stages of immunity using i.n./i.m. delivery of FPV-HIV/VVHIV control vaccine compared to a novel IL-13Rα2 adjuvanted HIV-vaccine (FPV-HIV IL-13Rα2/VV-HIV IL-13Rα) that has shown to induce excellent high avidity CD8⁺ T cells with greater protective immunity. The IL-13Rα2 adjuvanted vaccine induced greatly elevated HIV-specific memory CD8⁺IL-3⁺ and also CD8⁺IL-3⁺IFN-γ+ T cells compared to the control vaccine, where the expression in memory phase was greater than effector T cells. Both the control and IL- 13Rα2 adjuvanted vaccines, elicited elevated but similar numbers of antigen-specific GM-CSF⁺ memory CD8⁺ T cells. Data suggest that induction of both IL-3 and GM-CSF play a role in maintenance of antigen-specific memory CD8⁺ T cells which is linked to better protective immunity. These results also demonstrate that route of delivery, time post vaccination, expression kinetics/ length of antigen exposure, should not be neglected when evaluating the vaccine efficacy. Also IL-3/GM-CSF expression by memory CD8⁺ T cells could be a biomarker of protective immunity.