OUTER PERIARTERIOLAR LYMPHOID SHEATH ARREST AND SUBSEQUENT DIFFERENTIATION OF BOTH NAIVE AND TOLERANT IMMUNOGLOBULIN TRANSGENIC B-CELLS IS DETERMINED BY B-CELL RECEPTOR OCCUPANCY

T-dependent B cell responses in the spleen are initiated in the outer periarteriolar lymphoid sheath (PALS) and culminate in the generation of proliferative foci and germinal center reactions. By pulsing anti-h en egg lysozyme (HEL) immunoglobulin transgenic (IgTg) B cells with va rious concentrations of HEL in vitro before adoptive transfer into nor mal recipients, it was shown that a critical number of B cell receptor s (BCRs) must be ligated for B cells to undergo arrest in the outer PA LS. T cell help was manipulated independently of the BCR stimulus by i ncubating B cells expressing the appropriate major histocompatibility complex class II antigen with a peptide recognized by CD4(+) TCR Tg T cells. B cells which either failed to arrest in the outer PALS due to a subthreshold BCR stimulus, or arrested only transiently due to the b revity of the BCR stimulus, underwent an abortive response within the follicles when provided with T cell help. In contrast, naive B cells s timulated by a sustained, suprathreshold concentration of either forei gn or self-antigen and given T cell help, proliferated in the outer PA LS and then differentiated. Outer PALS arrest was not influenced by th e nature ofthe B cells occupying the follicle, but appeared to be dete rmined solely by the magnitude of BCR stimulation Thus antigen-pulsed B cells arrested in the outer PALS in an identical manner irrespective of whether the follicles comprised a population of normal B cells wit h multiple specificities, a monoclonal naive population, or a monoclon al population of tolerant B cells. In addition, tolerant B cells were found to relocate from the follicles to the outer PALS of HEL/anti-HEL double Tg mice in which the concentration of soluble self-antigen had been increased by zinc feeding. Similarly, when anti-HEL Tg mice were crossed with a second HEL Tg strain expressing a higher concentration of soluble HEL, the tolerant anti-HEL Tg B cells were located constit utively in the outer PALS. Thus, subtle variations in antigen concentr ation resulted in dramatic changes in positioning of B cells within th e spleen. A series of mixed bone marrow chimeras in which the effectiv e antigen concentration was inversely related to the number of self-re active B cells due to absorption of antigen by transgene-encoded membr ane and secreted Ig, was used to confirm that alteration in B cell pos ition previously attributed to changes in follicular composition could be explained on the basis of available antigen concentration, rather than the diversity ofthe repertoire.