TrkB mRNA and protein in mouse spleen: structure of the spleen of functionally deficient TrkB mice

Whereas it is nowadays clear that neurotrophins are involved in the regulation of various aspects of the functioning of immune system, knowledge of their actual immunomodulatory roles is still fragmentary and incomplete. In this respect, knock-out mouse models remain particularly unexplored. In the present study, the expression of the TrkB neurotrophin receptor in murine spleen was addressed at the mRNA (reverse transcription/polymerase chain reaction) and protein (Western blot) levels. Once the presence of TrkB at both levels was demonstrated, the age-dependent changes in the pattern of expression of the receptor were analyzed and quantified, and TrkB-positive cells were identified by immunohistochemistry. TrkB-immunoreactive cells, identified as red pulp macrophages, were detected in the spleen throughout postnatal development and adult life; their numbers peaked at the age of 15 days. The absence of functional TrkB did not appear to result in morphological changes as assessed by light and electron microscopy of spleens from 15-day-old mice knockout for the trkB gene. The present results support the idea that, in the murine spleen, TrkB and its ligands are involved in macrophage physiology in a developmentally regulated fashion, but they do not seem to be essential for macrophage survival.