Expression of the neurotrophin receptor TrkB in rat spleen macrophages

Increasing evidence suggests that some members of the family of the neurotr ophins could be involved in immune system functioning. Both neurotrophins a nd their tyrosine-kinase signal-transducing receptors, the so-called Trk re ceptors, have been detected in various lymphoid tissues in a number of spec ies. Nevertheless, their cellular localisation remains unclear in most case s. In this study, we used immunohistochemical techniques to localise TrkB i n the rat spleen (from 0 days to 2 years). Cells expressing TrkB-like immun oreactivity were found exclusively within the white pulp of the spleen, alo ng the marginal zone-follicle border and inside the follicles and periarter iolar lymphoid sheaths. These cells probably represented macrophage subpopu lations, since they expressed the ED3 rat macrophage antigen. No evidence o f TrkB-like protein expression in lymphocytes or follicular dendritic cells could be found. Furthermore, the density of TrkB-immunoreactive cells was observed to increase with age. Although the role of TrkB ligands in these c ells remains to be clarified, the present findings provide further evidence for the supposed role of neurotrophins in immune system homeostasis.