Immunohistochemical investigation of cells expressing CD21, membrane IgM, CD32 and a follicular dendritic cell marker in the lymphoid tissues of neonatal calves

Activation of B lymphocytes in the presence of passive maternal antibodies depends on expression of CD21, membrane IgM and CD32. On colligation with IgM, CD32 inhibits activation whereas CD21 enhances it. Recently, we assessed expression of CD21 and CD32 on IgM+ cells from lymphoid tissues of newborn calves by flow cytometry, but this approach does not provide information about spatial distribution within lymphoid compartments. Therefore, histologic sections of lymphoid tissues from newborn and 7-month-old calves were examined using an immunoperoxidase technique. In all calves, CD21 and IgM stained cells were collocated in the cortex and paracortex of the retropharyngeal lymph node, in the marginal zone of the spleen and in lymphoid aggregates of palatine tonsils. Most CD32+ cells were in the mantle zone of lymphoid follicles in 7-month-old calves, whereas only weak staining was observed in newborns. A few CD32+ cells were also observed in the paracortex at both ages. Absence of CD32+ cells in the center of follicles suggests that IgM+CD32− cells observed previously by flow cytometry were from germinal centers. Overall, there were few organized lymphoid aggregates within lymphoid tissues of newborn calves, and follicular dendritic cells were virtually undetectable. Their absence may be an important limitation for neonatal immunization.