ANTIGEN-DEPENDENT AND ANTIGEN-INDEPENDENT CA2-CELLS BY DENDRITIC CELLS COMPARED WITH B-CELLS( RESPONSES TRIGGERED IN T)

Dendritic cells (DCs) are much more potent antigen (Ag)-presenting cel ls than resting B cells for the activation of naive T cells. The mecha nisms underlying this difference have been analyzed under conditions w here ex vivo DCs or B cells presented known numbers of specific Ag-maj or histocompatibility complex (MHC) complexes to naive CD4(+) T cells h-om T cell antigen receptor (TCR) transgenic mice. Several hundred Ag -MHC complexes presented by B cells were necessary to elicit the forma tion of a few T-B conjugates with small contact zones, and the resulti ng individual T cell Ca2+ responses were all-or-none. In contrast, Ag- specific T cell Ca2+ responses can be triggered by DCs bearing an aver age of 30 Ag-MHC complexes per cell. Formation of T-DC conjugates is A g-independent, but in the presence of the Ag, the surface of the conta ct zone increases and so does the amplitude of the T cell Ca2+ respons es. These results suggest that Ag is better recognized by T cells on D Cs essentially because T-DC adhesion precedes Ag recognition, whereas T-B adhesion requires Ag recognition. Surprisingly, we also recorded s mall Ca2+ responses in T cells interacting with unpulsed DCs. Using DC s purified from MHC class II knockout mice, we provide evidence that t his signal is mostly due to MHC-TCR interactions. Such an Ag-independe nt, MHC-triggered calcium response could be a survival signal that DCs but not B cells are able to deliver to naive T cells.