Memory CD8+ T cells exhibit increased antigen threshold requirements for recall proliferation

Area of Science:

• Immunology
• T cell biology
• Cellular immunology

Background:

• Immunological memory allows rapid T cell recall responses upon antigen reencounter.
• Previous studies suggested memory T cells proliferate with lower antigen doses and co-stimulation requirements than naive T cells.

Purpose of the Study:

• To investigate the antigen threshold for cell cycle entry in naive and central memory CD8(+) T cells.
• To reconcile conflicting observations regarding memory T cell proliferation and antigen sensitivity.

Main Methods:

• In vivo proliferation assays comparing naive and memory T cells.
• Single-cell analysis of T cell receptor (TCR) signaling.
• Assessment of cell cycle effector molecules (Zap70, cMyc, p27).

Main Results:

• Naive T cells, but not memory T cells, proliferated in vivo upon limited antigen presentation.
• Both naive and memory T cells detected low-dose antigen, but only naive cells activated cell cycle effectors.
• Central memory T cells failed to activate Zap70, induce cMyc, or degrade p27 at antigen levels that triggered these in naive T cells.
• Reduced memory T cell sensitivity may stem from decreased surface TCR expression and increased protein tyrosine phosphatases.

Conclusions:

• Naive T cells initiate cell cycle progression at lower antigen thresholds than memory T cells.
• Memory T cells exhibit a novel, reduced sensitivity to antigen thresholds, potentially regulating recall expansion.
• Findings challenge existing paradigms of memory T cell recall responses.