CD4⁺ T cell-mediated neuroprotection is independent of T cell-derived BDNF in a mouse facial nerve axotomy model

Background: The production of neurotrophic factors, such as BDNF, has generally been considered an important mechanism of immune-mediated neuroprotection. However, the ability of T cells to produce BDNF remains controversial. Methods: In the present study, we examined mRNA and protein of BDNF using RT-PCR and western blot, respectively, in purified and reactivated CD4⁺ T cells. In addition, to determine the role of BDNF derived from CD4⁺ T cells, the BDNF gene was specifically deleted in T cells using the Cre-lox mouse model system. Results: Our results indicate that while both mRNA expression and protein secretion of BDNF in reactivated T cells were detected at 24h, only protein could be detected at 72h after reactivation. The results suggest a transient up-regulation of BDNF mRNA in reactivated T cells. Furthermore, in contrast to our hypothesis that the BDNF expression is necessary for CD4⁺ T cells to mediate neuroprotection, mice with CD4⁺ T cells lacking BDNF expression demonstrated a similar level of facial motoneuron survival compared to their littermates that expressed BDNF, and both levels were comparable to wild-type. The results suggest that the deletion of BDNF did not impair CD4⁺ T cell-mediated neuroprotection. Conclusion: Collectively, while CD4⁺ T cells are a potential source of BDNF after nerve injury, production of BDNF is not necessary for CD4⁺ T cells to mediate their neuroprotective effects.