Age related variation in expression of CD21 and CD32 on bovine lymphocytes: a cross-sectional study.

It is difficult to induce active immune responses in neonatal calves, partly due to limited functional ability of the immune system and partly due to immune inhibitory effects of maternal antibodies. CD21 (complement receptor 2), an activating receptor, and CD32 (Fc gamma receptor II), an inhibitory receptor, can both be expressed by mature B lymphocytes. Studies of the signalling pathways regulating B cell activation suggest that these receptors have mutually antagonistic effects, that may determine immune responsiveness in the first months of life. In a cross-sectional study, blood was collected from 41 Holstein calves, 1-90 days of age, and 12 mature cows. The absolute number of CD21 and CD32 positive cells increased from birth until 90 days of age, with CD21+ cells showing a greater relative increase compared to CD32+ cells. Approximately 89% of CD21+ cells also expressed CD32. In calves, CD32+ cells consisted of two distinct populations, characterized to be CD14+CD32+IgM(-) (44%) and CD14(-)CD32+IgM+ (53%) cells, consistent with monocytes and B cells respectively. Mean fluorescence intensity (MFI) for CD21 did not change appreciably up to 90 days of age, and mean values were slightly lower than for adults. MFI for CD32 on CD21+ lymphocytes increased slightly over the first 3 months of life, but values were lower than for adults. Over 92% of calf membrane immunoglobulin M+ (mIgM) B cells expressed CD21, however only 60-80% of CD21+ cells were mIgM positive, regardless of age. Although the CD21+IgM(-) cells were not characterized further, it was evident that CD21 is not a suitable surrogate marker for B lymphocytes in calves. Most importantly this study showed that from birth, the majority of circulating B cells express both CD21 and CD32. This suggests that these cells are subject to activating and inhibiting influences mediated by these receptors from birth. Expression of CD21 does not appear to be a limiting factor in neonatal B cell responses.