Response of U n p r i m e d CBA Mice to 4 - Hydroxy - 3 - Iodo - 5 -

Cells of the immune system are heterogeneous by function, origin, and physical properties and there is increasing interest in isolation and characterization of the various lymphocyte subsets. Previous studies in this laboratory have made use of a battery of physical separation methods (cell electrophoresis, adherence column fractionation, sedimentation velocity separation, and buoyant density separation) to characterize the bone marrow-derived, bursal-equivalent (B) lymphocyte population of the adult CBA mouse, and have shown this population to be heterogeneous by all physical criteria investigated (1). These physically distinct B-lymphocyte subpopulations are being investigated in the context of the B-cell pathway of development, and their place in this sequence. The newborn, functional B lymphocyte bearing a full complement of immunoglobulin receptors develops from a multipotent hemopoietic stem cell (2-6), probably via some intermediate "pre-B"-cell stage. Lafleur et al. have studied this maturation sequence using cell separation methodology (7-9), while Osmond and Nossal have investigated the phase of B-cell development in a kinetic study of marrow B-lymphocyte production (10). Both approaches suggested that the functional antigen-inexperienced "virgin" B cell is a typical small lymphocyte. Earlier studies from this laboratory, however, seemed to indicate that typical small lymphocytes of the adult CBA mouse were largely memory cells, the virgin B cells constituting a minor, atypical subset of B cells in terms of surface properties (11, 12). In contrast to the finding of Lafleur et al. of slowly sedimenting functional B cells (7-9), Strober has recently reported unprimed antibody-forming cell (AFC) 1 progenitor activity among rapidly sedimenting