Differential requirements of CD4+ T‐cell signals for effector cytotoxic T‐lymphocyte (CTL) priming and functional memory CTL development at higher CD8+ T‐cell precursor frequency

Increased CD8⁺ T‐cell precursor frequency (PF) precludes the requirement of CD4⁺ helper T (Th) cells for primary CD8⁺ cytotoxic T‐lymphocyte (CTL) responses. However, the key questions of whether unhelped CTLs generated at higher PF are functional effectors, and whether unhelped CTLs can differentiate into functional memory cells at higher PF are unclear. In this study, ovalbumin (OVA) ‐pulsed dendritic cells (DC_OVA) derived from C57BL/6, CD40 knockout (CD40^?/?) or CD40 ligand knockout (CD40L^?/?) mice were used to immunize C57BL/6, Ia^b?/?, CD40^?/? or CD40L^?/? mice, whose PF was previously increased with transfer of 1 × 10⁶ CD8⁺ T cells derived from OVA‐specific T‐cell receptor (TCR) transgenic OTI, OTI(CD40^?/?) or OTI(CD40L^?/?) mice. All the immunized mice were then assessed for effector and memory CTL responses. Following DC immunization, relatively comparable CTL priming occurred without CD4⁺ T‐cell help and Th‐provided CD40/CD40L signalling. In addition, the unhelped CTLs were functional effectors capable of inducing therapeutic immunity against established OVA‐expressing tumours. In contrast, the functional memory development of CTLs was severely impaired in the absence of CD4⁺ T‐cell help and CD40/CD40L signalling. Finally, unhelped memory CTLs failed to protect mice against lethal tumour challenge. Taken together, these results demonstrate that CD4⁺ T‐cell help at higher PF, is not required for effector CTL priming, but is required for functional memory CTL development against cancer. Our data may impact the development of novel preventive and therapeutic approaches in cancer patients with compromised CD4⁺ T‐cell functions.