GM-CSF production by CD4⁺ T cells in MS patients: Regulation by regulatory T cells and vitamin D

Background/objective: Data from animal models of MS suggest that GM-CSF⁺CD4⁺T cells are pathogenic cells. Therefore, GM-CSF production by CD4⁺T cells of MS patients and their susceptibility to regulatory mechanisms were investigated. Methods: Intracellular flowcytometry was performed to determine the GM-CSF⁺CD4⁺T cell fraction in PBMC and CSF of MS patients and controls. The effect of regulatory T cells (Tregs) on GM-CSF production by CD4⁺T cells was studied in MS patients using a proliferation-suppression assay. Finally, GM-CSF⁺CD4⁺T cell fraction and GM-CSF protein levels in supernatant were assessed in anti-CD3-stimulated CD4⁺T cell cultures derived from healthy controls and MS patients, in the presence or absence of the active vitamin D metabolite calcitriol. Results: The GM-CSF⁺CD4⁺T cell fraction in the peripheral blood did not differ between controls and MS patients. This T cell population could also be detected in the CSF of both subjects with MS as well as subjects with another diagnosis. In the CSF, it comprised a significant fraction of the T cell population. Upon in vitro stimulation of PBMC with anti-CD3 antibody, no differences were observed in GM-CSF⁺CD4⁺T cell frequencies. GM-CSF secretion was susceptible to regulation by Treg and vitamin D. Suppression of GM-CSF secretion by vitamin D was reduced in MS patients. Conclusions: Our study showed no elevation in GM-CSF⁺CD4⁺T cell fractions in MS patients compared to controls. Furthermore, GM-CSF secretion was prone to regulation by Treg and vitamin D, the latter being less effective in MS patients.