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Background:Tuberculosis is a leading cause of morbidity and mortality in humans worldwide.There is an urgent need for new and effective drugs to treat tuberculosis and shorten the duration of tuberculosis therapy.1,25-dihydroxy vitamin D3 (1,25 (OH)2D3) has been reported to have a synergistic effect with pyrazinamide (PZA) in killing tubercle bacilli in vitro.The addition of 1,25 (OH)2D3 to standard tuberculosis treatment should benefit patients if the adjunctive drug has a synergistic effect in vivo.Thus,in this study,calcitriol (bioactive 1,25 (OH)2D3) was administered to mice undergoing treatment for Mycobacterium tuberculosis (M.tb) infection with PZA,a first-line anti-tuberculosis drug,to determine whether vitamin D3 enhances the therapeutic effect.Methods:C57BL/6 female mice were infected with the M.tb H37Rv strain through aerosol exposure.Calcitriol and PZA,either alone or in combination,were orally administered to the M.tb infected mice.The effect of calcitriol on PZA activity was determined by evaluating the bacterial burden and analyzing the histopathological lesions in the lungs and spleen.To investigate the expression of inflammatory cytokines and anti-microbial peptide genes,we determined the transcriptional levels of interferon-γ (IFN-γ),interleukin-4 (IL-4),mouse β-defensin-2 (mBD2),and cathelicidin LL-37 through real-time quantitative polymerase chain reaction.The protein levels of IFN-γ were detected by enzyme-linked immunosorbent assay.Differences between groups were analyzed with independent samples t-test or one-way analysis of variance.Results:Calcitriol alone had little effect on tuberculosis infection,whereas PZA,compared with saline control treatment,decreased the bacterial burden (spleens:PZA vs.saline,4.82 ± 0.22 vs.5.22 ± 0.40 Log10 colony-forming units [CFU]/gram,t =2.13,P ＜ 0.05;lungs:PZA vs.saline,5.55 ± 0.15 vs.6.83 ± 0.46 Log10 CFU/gram,t =6.56,P ＜ 0.01) and pathological lesions in the lungs.Simultaneous administration of calcitriol with PZA,compared with PZA alone,decreased the bacterial load (spleen:calcitriol + PZA vs.PZA,4.37 ± 0.13 vs.4.82 ± 0.22 Log10 CFU/gram,t =4.36,P ＜ 0.01;lung:calcitriol + PZA vs.PZA,5.03 ± 0.32 vs.5.55 ± 0.15 Log10 CFU/gram,t =3.58,P ＜ 0.01) and attenuated the lung lesions (gross pathological score:calcitriol + PZA vs.PZA,3.25 ± 0.50 vs.2.50 ± 0.58,t =1.96,P ＜ 0.05;affected area of total lung area:calcitriol + PZA vs.PZA,30.75％ ± 6.50％ vs.21.55％ ± 2.99％,t =2.66,P ＜ 0.05).Further studies demonstrated calcitriol significantly increased the expression of anti-inflammatory cytokine IL-4 but suppressed production of the pro-inflammatory cytokine IFN-γ (IL-4:calcitriol vs.saline,5.69 ± 0.50 vs.2.80 ± 0.56 fold of control,t=6.74,P ＜ 0.01;IFN-γ:calcitriol vs.saline,1.36 ± 0.11 vs.4.13 ± 0.83 fold of control,t=5.77,P ＜ 0.01).In addition,calcitriol alone or in combination with PZA significantly enhanced the transcriptional level of anti-microbial peptides (cathelicidin LL-37:calcitriol vs.saline,10.59 ± 1.03 vs.2.80 ± 0.90 fold of control,t =9.85,P ＜ 0.01;mBD2:calcitriol vs.saline,7.92 ± 0.62 vs.1.79 ± 0.45 fold of control,t =13.82,P ＜ 0.01),whereas PZA exerted a negative effect on anti-microbial peptide gene expression.Conclusions:Calcitriol as adjunctive treatment can result in beneficial treatment outcomes in M.tb infection by suppressing the inflammatory response and up-regulating the expression of anti-microbial peptides.These results indicate the feasibility of using calcitriol adiunctively with standard chemotherapy for the treatment of M.tb infection.