Ontogenic characterization of thymic B lymphocytes. Analysis in different mouse strains

We have characterized a population of murine B lymphocytes present in the thymus (TBL). They are a minor subset (0.2%−1% of total thymocytes), present from perinatal periods onwards and constituted by activated cells with a high proportion of Ig‐secreting cells. They represent the first B lymphocytes detected that secrete IgG after birth. Functional analysis reveals that the frequency of lipopolysaccharide‐responding cells in TBL is 5‐ to 10‐fold lower than in the spleen. TBL from adult mice did not show any significant difference in their VH repertoire expression when compared to peripheral B lymphocytes. Further‐ more, we have been able to isolate a subpopulation of B220+IgM−CD3− thymocytes whose putative B cell precursor potential needs to be directly analyzed. These and other findings support the intrathymic resident characteristics of TBL and suggest new ways of elucidating its physiological role in the complex selective processes occurring inside the thymus.