CD57, A MARKER FOR B-CELL ACTIVATION AND SPLENIC ELLIPSOID-ASSOCIATEDRETICULAR CELLS OF THE CHICKEN

We have demonstrated that the ellipsoid-associated reticular cells of chicken spleen express CD57, a marker for B-cell activation. These cel ls are characterised by their spindle-shaped morphology, tissue distri bution and the absence of certain leucocyte-specific markers. They are phagocytotic and possess high endogenous non-specific esterase activi ty. Previous reports failed to detect CD57 expression on ellipsoid-ass ociated reticular cells, probably because the tissue sections were dif ferently treated before immunohistochemistry. CD57 is also expressed b y a small number of T-cells in the spleen and the caecal tonsils. This number is highly variable between individual chickens depending on th e activation state of the immune system. Moreover, CD57 is expressed b y bursal lymphocytes (90% or more) but not by B-cells of the periphera l blood. More interestingly, we have been able to discriminate and qua ntify three B-cell populations of the secondary lymphoid organs, i.e. resting B-cells, germinal centre B-cells and plasma cells, based on th eir expression levels of CD57 and Bu-1 (a pan B-cell marker). Thus, CD 57 should be considered as a B-cell activation marker, rather than as a marker for bursal B-cells; it is also a valuable marker for the immu nohistochemical study of ellipsoid-associated reticular cells of chick en spleen.