Targeting Antigen to Clec9A Primes Follicular Th Cell Memory Responses Capable of Robust Recall

Targeting Ags to dendritic cell (DC) surface receptors can induce a variety of responses depending on the DC type targeted, the receptor targeted, and the adjuvant used. Clec9A (DNGR-1), which is expressed by CD8 + DCs, has been shown to bind F-actin exposed on damaged cells. Targeting Ag to this receptor in mice and nonhuman primates induces strong humoral immunity even in the absence of adjuvant, a process seen for a few select DC receptors. In contrast with other receptors, however, targeting Clec9A induces long-lived, affinity-matured Ab responses that are associated with efficient CD4 + T cell responses shown to possess properties of follicular Th cells (T FH ). In this article, we provide definitive evidence that Clec9A targeting promotes the development of T FH by showing that responding CD4 T cells express CXCR5, PD1, the T FH transcription factor Bcl6, and the cytokine IL-21, and that these cells localize to germinal centers. Furthermore, we extend studies from the model Ag OVA to the viral Ag glycoprotein D of HSV-1 and examine the capacity of primed T FH to form functional memory. We show that targeting glycoprotein D to Clec9A even in the absence of adjuvant induced long-lived memory CXCR5 + PD1 hi CD4 + T cells that proliferated extensively upon secondary challenge and rapidly developed into effector T FH . This was associated with enhanced germinal center B cell responses and accelerated Ab production. Our study indicates that targeting Ags to Clec9A in the absence of adjuvant routinely generates T FH responses that form long-lived memory capable of robust secondary T FH responses.