The Memory Inflation Response Against Spread-Defective CMV Requires Priming-Independent CD4⁺ T Cell Help

Memory inflation (MI) is a distinctive CD8⁺ T cell response to chronic cytomegalovirus (CMV) infection, characterized by non-contracting, functional populations that are predominantly effector differentiated. Harnessing such responses has received growing attention as a promising vaccination strategy, using spread-defective CMV-vectors. However, many underlying mechanistic aspects of CD8⁺ MI responses, including their dependency on CD4⁺ T cell help, remain unclear. Using a Rag KO transfer model, we demonstrate that CD4⁺ T cells enhance effector differentiation of inflationary CD8⁺ T cells when present during priming, or when introduced at late timepoints post-infection, indicating a continuous, priming-independent support mechanism. This helper requirement was specific to spread-defective CMV infection, whereas spread-competent virus induced MI even in the absence of CD4⁺ T cells. We confirmed the importance of continuous support in wild-type mice, where late CD4 depletion attenuated an established MI response. Gene set enrichment analysis implicates IFNγ, TNF, and IL-1 signaling as potential mediators of CD4⁺ T cell support. Our findings clarify CD4⁺ T cell help requirements for maintaining effector populations in MI during chronic antigen exposure. These results have particular relevance for spread-defective CMV-based vaccination strategies and T cell-based therapies in which the persistence and expansion characteristic of MI are desirable qualities.