Cluster formation among small resting B lymphocytes leading to B cell activation.

Intercellular contacts are thought to be a crucial event in various aspects of immune responses. We have recently suggested that the self class II MHC-restricted B-B cell interaction is a prerequisite process in the polyclonal B cell differentiation induced by a novel T cell-derived lymphokine B151-TRF2 or LPS. The results have led us to the possibility that such a class II MHC-restricted B-B cell interaction promotes the formation of B cell clusters responsible for the development of IgM-producing cells upon subsequent stimulation with the polyclonal B cell activator. To examine directly this possibility, the present study has utilized a recently devised ACAS470 work station capable of sorting out desired cells in a tissue culture plate in situ. Clusters and non-clusters were observed after 2 day preculture of murine resting B cells in the absence of the polyclonal B cell activator. Interestingly, B cells in clusters purified by the ACAS470 gave rise to IgM-producing cells when stimulated with B151-TRF2 or LPS for an additional 2 days, whereas non-clustered B cells failed to differentiate. Moreover, such a B cell activation was not observed when the 2 day preculture of the resting B cells was conducted in the presence of anti-class II MHC mAb but not of anti-class I MHC mAb. Thus, the present results support the notion that cluster formation among small resting B cells accompanying the class II MHC-restricted B-B cell interaction makes the B cells responsive to differentiation signals of polyclonal B cell activator.