Analysis of adjuvant function by direct visualization of antigen presentation in vivo: Endotoxin promotes accumulation of antigen-bearing dendritic cells in the T cell areas of lymphoid tissue

T cell activation requires exposure to processed Ag and signaling by cytoki nes and costimulatory ligands, Adjuvants are thought to enhance immunity pr imarily through up-regulation of the latter signals, Here, we explore the e ffect of the bacterial adjuvant, endotoxin, on Ag presentation by B cells a nd dendritic cells (DC), Using an mAb (C4H3) specific for the hen egg lysoz yme (HEL) 46-61 determinant bound to I-A(k), we analyze processed Ag expres sion and the tissue distribution of presenting cells following systemic adm inistration of soluble HEL to mice. In both LPS-responsive and -hyporespons ive mice given endotoxin-containing HEL, B cells rapidly display surface 46 -61/I-A(k) complexes. In marked contrast, in LPS-hyporesponsive mice, splen ic DC show little gain in C4H3 staining. In LPS-responsive animals, interdi gitating DC in T cell areas show no staining above background at early time s after HEL administration, but C4H3(+) DC rapidly accumulate in the outer periarteriolar lymphoid sheaths (PALS) and in follicular areas. Within a fe w hours, C4H3(+) DC appear in the T cell areas,concomitant with a decline i n C4H3(+) cells in the outer PALS, suggesting migration between these two s ites. Endotoxin enhancement of C4H3 staining is seen for both CD8 alpha(-) and CD8 alpha(+) DC subsets. These data suggest that a major effect of adju vants is to promote mobilization of Ag-bearing DC to the T areas of lymphoi d tissue, and possibly also to enhance Ag processing by these DC. Thus; mic robial products promote T cell immunity not only through DC activation for cosignaling,,but through improvement in signal 1 delivery.