OBJECTIVES: It has been shown that calcitriol and all-trans retinoic acid (ATRA) 
have modulatory effects on the immune system. The present study investigates the 
synergistic effects of combination treatment of calcitriol and ATRA in 
experimental autoimmune encephalomyelitis (EAE), an animal model of multiple 
sclerosis (MS).
METHODS: The mice were allocated to four preventive groups, each consisting of 
eight animals, ATRA (250 μg/mouse), calcitriol (100 ng/mouse), combination of 
ATRA and calcitriol (125  μg/mouse and 50 ng/mouse) and vehicle groups. EAE was 
induced by MOG35-55 peptide in female C57BL/6 mice. Treatments were initiated at 
day 1 before immunization and continued every other day throughout the study 
until the day 21 post-immunization. Splenocytes were isolated from EAE-induced 
mice and the expression of retinoic acid receptor-related orphan receptor gamma 
t (ROR-γt), Interleukin-17 (IL-17), transforming growth factor beta (TGF-β), and 
forkhead box P3 (FOXP3) genes was measured using real-time polymerase chain 
reaction.
RESULTS: The expression of FOXP3 and TGF-β genes in the splenocytes of 
combination-treated and calcitriol alone-treated mice was significantly 
increased compared to vehicle group (P < 0.05). The expression of ROR-γt and 
IL-17 genes in the splenocytes of ATRA, calcitriol and combination- treated mice 
was significantly reduced compared to those of vehicle- treated mice (P < 0.05). 
The relative expression level of ROR-γt was significantly (P < 0.05) lower in 
the combination group than in the mice treated by ATRA or calcitriol alone.
DISCUSSION: This study demonstrated that treatment with combination of 
calcitriol and ATRA can be considered as a new strategy for MS prevention and 
treatment.