Background <p>Immune checkpoint molecules such as lymphocyte activation gene-3 (LAG3) play a critical role in modulating host–pathogen interactions during chronic parasitic infections; however, their functions in early infection remain poorly defined. Using a murine model infected with <i>Echinococcus multilocularis</i> (<i>E. multilocularis</i>), a lethal helminth causing alveolar echinococcosis (AE), we elucidate the stage-specific regulatory functions of LAG3 in CD4⁺ T cell immunity during early infection.</p> Methods <p><i>Echinococcus multilocularis</i>-infected mice were employed as the experimental model. Flow cytometry was used to analyze LAG3 expression on CD4⁺ T cell subsets and the production of intracellular cytokines. The in vivo functional role of CD4⁺ T cells was further investigated using LAG3-knockout mice and adoptive T cell transfer models. Infected wild-type mice received LAG3 blocking antibody treatment, with parasite burden assessed by measuring metacestode weight. Hepatic pathology was evaluated using hematoxylin and eosin and Masson’s trichrome staining.</p> Results <p>We found that LAG3 was predominantly expressed on hepatic effector CD4⁺ T (CD4⁺ Teff) cells, particularly the CD69⁺ subset in the early stages of <i>E. multilocularis</i> infection. Functional analysis showed that LAG3-expressing CD4⁺CD44⁺ T cells secreted elevated levels of interleukin-4 (IL-4) and IL-10 at 2 and 4&#xa0;weeks post-infection. LAG3 deficiency further enhanced the production of interferon-y (IFN-γ), IL-4, and IL-10 by splenic CD4⁺ T cells in the initial infection phase, which may contribute to the slight suppression of <i>E. multilocularis</i> growth and development observed in the livers of LAG3-knockout mice. In an adoptive transfer model of early&#xa0;<i>E. multilocularis</i>&#xa0;infection, LAG3^−/−CD4⁺ T cells exhibited a greater propensity to differentiate into CD4⁺ effector T (Teff) cells and produced higher levels of IFN-γ and IL-10 in both the livers and spleens of recipient mice. Finally, early administration of anti-LAG3 monoclonal antibody (mAb) reduced metacestode burden—though the change was not statistically significant—and concurrently exacerbated hepatic inflammation and fibrosis.</p> Conclusions <p>Our study reveals a previously unrecognized role for LAG3 as an immune checkpoint during early <i>E. multilocularis</i> infection, highlighting its function in limiting anti-parasitic CD4⁺ T cell responses. Thus, while timed LAG3 blockade may represent a potential therapeutic strategy for AE, our data underscore the critical importance of stage-specific intervention to balance parasitic clearance with the control of fibrosis and tissue damage.</p> Graphical Abstract <p></p>