Background <p>Diabetic cardiomyopathy (DCM) is one of the severe complications in type 1 diabetes mellitus (T1DM) patients with impaired cardiac function and faster progression to heart failure due to systolic malfunction. It has been demonstrated that macrophage-mediated chronic inflammation is closely associated with DCM. Cell division cycle 42 (Cdc42) plays a crucial role in regulating the polarization, migration and phagocytosis of macrophages, however, the underlying mechanism of Cdc42 in DCM remains to be elucidated.</p> Methods <p>Mouse DCM models with T1DM were generated using myeloid-specific Cdc42-knockout (Cdc42^<i>Mye</i>) and Cdc42^{<i>Flox/Flox</i>} (Cdc42^<i>Flox</i>) male mice at 10–12 weeks old by injection of streptozotocin (STZ, 50&#xa0;mg/kg/day) for 6 continuous days and then following the observation of 16 weeks. Cardiac functions were assessed by echocardiography in vivo, and cardiac morphological and histopathological alterations were evaluated by hematoxylin and eosin (HE), Masson’s trichrome, Picrosirius red, and immunohistochemistry staining, respectively. The infiltration of myeloid macrophages was examined by multiplex immunofluorescence tyramide signal amplification (TSA) assay or Rosa mTmG fluorescent myeloid tracking reporter in mice. Macrophage pyroptosis, cardiomyocyte damage and myofibroblast activation were evaluated by Western blot and immunofluorescence analysis. Additionally, single-cell RNA-seq (scRNA-seq) data analysis was performed to explore macrophage-mediated signaling in DCM using publicly available GSE datasets from STZ-induced T1DM mice.</p> Results <p>Myeloid Cdc42 deletion exacerbated T1DM-induced cardiac dysfunctions and histopathological changes including cardiac fibrosis, and promoted T1DM-induced macrophage infiltration and M1 polarization of macrophages, and facilitated T1DM-induced pyroptosis by activating NLRP3 inflammasome in the hearts in mice. Notably, there were no significant differences between Cdc42^<i>Mye</i> and Cdc42^<i>Flox</i> mice under normal condition. In addition, gene set enrichment analysis (GSEA) of scRNA-seq data indicated that low Cdc42 expression in macrophages was positively associated with the NF-κB signaling pathway in DCM. Mechanistically, we demonstrated that Cdc42 deficiency or inhibition aggravated T1DM-induced cardiomyocyte injury and cardiac fibrosis by activating ERK_1/2-NF-κB-interleukin-1β signaling pathway-mediated macrophage pyroptosis.</p> Conclusions <p>This study demonstrates that myeloid Cdc42 deficiency or inhibition aggravates T1DM-induced cardiomyocyte injury and cardiac fibrosis of DCM by promoting macrophage pyroptosis and inflammatory responses, which might provide a novel therapeutic target for immunomodulatory intervention in DCM of type 1 diabetes mellitus.</p> Graphical abstract <p></p>