B-1-LIKE CELLS EXIST IN SHEEP - CHARACTERIZATION OF THEIR PHENOTYPE AND BEHAVIOR

Two populations of B lymphocytes, B-l (CD5(+) and/or CD11b(+)) and B-2 (CD5(-) and CD11b(-)) cells have been described. In mice, which is th e species of reference for B-l and B-2 cell studies, these two subsets present different developmental schemes, phenotypes, antibody reperto ires, localization and behaviours. Interestingly, in sheep, B cells re arrange their immunoglobulin (Ig) loci around the neonatal period, sim ilarly to murine B-l cells. However, the phenotype of the sheep B cell s has not been characterized with regards to their developmental pathw ay. In this report, we show that two sheep B-cell subsets can be disti nguished on the basis of CD11b expression. Relative to CD11b(-) B cell s, the CD11b(+) B cells frequently co-express CD5, CD11c, higher level s of surface IgM (sIgM), show larger cell size and higher cell-cycling activity, and thus present a B-l-like phenotype. However, unlike muri ne B-l cells, sheep B-l like cells mainly localize in blood, display a higher propensity to spontaneous apoptosis relative to B-2-like cells , and proliferate after sIgM stimulation. Our data show that despite n eonatal immunoglobulin loci rearrangements, sheep B cells do not all e xpress a B-l-like phenotype. However, B-l-and B-2-like cells co-exist and present phenotypic and behavioural specificities. Nevertheless, sh eep B-l-and B-2-like cells differ from the murine B-l and B-2. cells i n their cell behaviour. These subsets can thus not be considered as tr ue homologues among species.