Background <p>Glioblastoma (GBM) is an immunologically “cold” tumor in which S100A4-driven immunosuppressive myeloid cells promote progression and resistance. However, the specific immune subpopulation that intrinsically expresses S100A4 and the corresponding cell surface targets remain poorly defined.</p> Materials and methods <p>S100A4 expression was profiled across immune subsets in healthy human bone marrow and peripheral blood by flow cytometry. TCGA GBM datasets were used to identify S100A4-correlated surface markers, and CD13 (encoded by <i>ANPEP</i>) was validated across the Human Protein Atlas, Immgen, GEO and single-cell RNA-seq datasets. The functional link between CD13, S100A4, and TGFβ was assessed by network analysis and correlation. Drug response analysis was conducted using GEPIA 3 platform.</p> Results <p>Monocytes were the predominant S100A4-expressing population in both bone marrow and peripheral blood. CD13 emerged as the surface marker most significantly correlated with S100A4 in GBM. CD13⁺ monocytes co-expressed high levels of S100A4 and were transcriptionally positioned between monocytic and granulocytic lineages. These cells displayed a strong association with TGFβ signaling, suggesting a CD13-TGFβ-S100A4 immunosuppressive axis. Notably, temozolomide conferred a significant survival benefit specifically in CD13-high GBM patients.</p> Conclusions <p>CD13 marks S100A4-high immunosuppressive monocytes in GBM and represents an accessible surface target for monitoring and potentially depleting this pathogenic population. The CD13-S100A4-TGFβ axis and the predictive value of CD13 for temozolomide response offer a translational framework for advancing GBM immunotherapy.</p>