Infection with the cestodeEchinococcus multilocularis(E.multilocularis) causes alveolar echinococcosis (AE), a tumor-like disease predominantly affecting the liver but able to spread to any organ. T cells develop functional defects during chronicE.multilocularisinfection, mostly due to upregulation of inhibitory receptors such as T-cell immunoreceptor with immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domains (TIGIT) and programmed death-1 (PD-1). However, the role of lymphocyte activation gene-3 (LAG3), an inhibitory receptor, in AE infection remains to be determined. Here, we discovered that high expression of LAG3 was mainly found in CD4+T cells and induced regulatory T cells (iTregs) in close liver tissue (CLT) from AE patients. In a mouse model ofE.multilocularisinfection, LAG3 expression was predominantly found in T helper 2 (Th2) and Treg subsets, which secreted significantly more IL-4 and IL-10, resulting in host immune tolerance and disease progression at a late stage. Furthermore, LAG3 deficiency was found to drive the development of effector memory CD4+T cells and enhance the type 1 CD4+T-cell immune response, thus inhibiting metacestode growthin vivo. In addition, CD4+T cells from LAG3-deficient mice produced more IFN-γ and less IL-4 when stimulated byE.multilocularisprotoscoleces (EmP) antigenin vitro. Finally, adoptive transfer experiments showed that LAG3-knockout (KO) CD4+T cells were more likely to develop into Th1 cells and less likely to develop into Tregs in recipient mice. Our work reveals that high expression of LAG3 accelerates AE disease progression by modulating the immune imbalance of CD4+T-cell subsets. These findings may provide a novel immunotherapeutic strategy againstE.multilocularisinfection.