Abstract

CD40 ligand (CD40L) is essential for the establishment of humoral and cellular immunity. Although its functions are well characterized, little is known about the regulation of its surface expression because of CD40-mediated modulation. Intracellular staining shows that most CD40L in resting, effector CD4 T cells is intracellular (iCD40L). We used a surface mobilization assay, in which the detection antibody is included during stimulation to enhance sensitivity, to show that iCD40L comes to the cell surface within 30min upon stimulation in a cycloheximide-insensitive manner. Microscopic analysis demonstrated that iCD40L is co-localized with Fas ligand but not with CTLA-4 in secretory lysosomes. We also found that LCMV-specific effector and memory Th1 cells have iCD40L, which comes to the cell surface upon antigen recognition. Finally, we observed that both naïve and effector/memory CD4 T cells from unmanipulated animals contain iCD40L, while only effector/memory T cells exhibit rapid surface mobilization of iCD40L. Surface mobilization of iCD40L is observed in T cells from CD40-deficient mice, indicating that CD40 is not required for sorting of CD40L to secretory lysosomes. Our results suggest that effector CD4 T cells can provide CD40-mediated stimulation to antigen bearing APCs by delivering iCD40L during transient interactions.

This work was supported by Grant AI29544 from the NIH.

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