ObjectiveTo develop a novel oral delivery system for interleukin12 (IL12) using genetically engineered Bifidobacterium longum(B. longum) as the carrier and further evaluate the efficacy of IL12expressed B. longum on the coxsackievirus B3 (CVB3)induced myocarditis in mice.MethodsA mIL12 gene expression vector pBBADsIL12 for B. longum was constructed and transformed into Bifidobacterium, the expression of mIL12 in the engineered B. longum was identified in vitro by Western Blot and enzymelinked immunosorbent assay (ELISA) after Larabinose induction. BLAB/c mice were inoculated i.p. with infectious dose of CVB3 for fourteen days and were divided randomly into three groups. The IL12 group and green fluorescent protein group (GFP group) were orally administered with pBBADsIL12 and pBBADsGFP transformed B. longum for fourteen days respectively after the inoculation of the virus； saline group was administered i.p. with sterile PBS. All animals were killed in day 14 of treatment, and the murine hearts were dissected aseptically for hematoxylineosin (HE) staining, viral titer and RNA extraction for Th1 cell cytokines quantification.ResultsAfter 14 days of treatment, HE staining revealed that the severity of virusinduced myocarditis in IL12 group was reduced compared with that of GFP group and saline group; the percentage of cardiac pathological lesions and CVB3 titers in IL12 group was （18±5）% and (2.89±0.10)pfu/g respectively,which was significantly lower than that of GFP group (［31±6］%, ［4.83±0.14］pfu/g) and saline group (［32±9］%, ［4.80±0.15］pfu/g), respectively (all P<0.01); levels of interferonγ(IFNγ) and tumor necrosis factorα(TNFα) in cardiac tissue and supernatants of IL12 group was（2.27±0.15）pg/mL and （3.05±0.17）pg/mL, respectively, which was significantly higher than that of GFP group (［1.32±0.11］pg/mL,［2.37±0.16］pg/mL) and saline group (［1.38±0.11］pg/mL, ［2.37±0.12］pg/mL）, respectively (all P<0.01).ConclusionA novel oral delivery system of Bifidobacterium for murine IL12 has been successfully established. Oral administration of mIL12transformed B. longum may play a therapeutic role in the treatment of CVB3induced myocarditis in the mice.