Tumor-associated macrophages (TAMs) represent alternatively activated (M2) macrophages that support tumor growth. Previously, we have described a special LYVE-1(+) M2 TAM subset in vitro and in vivo; gene profiling of this TAM subset identified MS4A8A as a novel TAM molecule expressed in vivo by TAM in mammary carcinoma and malignant melanoma. In vitro, Ms4a8a mRNA and MS4A8A protein expression was strongly induced in bone marrow-derived macrophages (BMDMs) by combining M2 mediators (IL-4, glucocorticoids) and tumor-conditioned media (TCM). Admixture of MS4A8A(+) TCM/IL-4/GC-treated BMDM significantly enhanced the tumor growth rate of subcutaneously transplanted TS/A mammary carcinomas. Upon forced overexpression of MS4A8A, Raw 264.7 macrophage-like cells displayed a special gene signature. Admixture of these MS4A8A(+) Raw 264.7 cells also significantly enhanced the tumor growth rate of subcutaneously transplanted mammary carcinomas. To identify the signaling pathways involved in synergistic induction of MS4A8A, the major signaling cascades with known functions in TAM were analyzed. Although inhibitors of NF-B activation and of the MAPK JNK and ERK did not show relevant effects, the p38/ MAPK inhibitor SB203580 strongly and highly significantly (p > 0.001) inhibited MS4A8A expression on mRNA and protein level. In addition, MS4A8A expression was restricted in M2 BMDM from mice with defective GC receptor (GR) dimerization indicating that classical GR gene regulation is mandatory for MS4A8A induction. In conclusion, expression of MS4A8A within the complex signal integration during macrophage immune responses may act to fine tune gene regulation. Furthermore, MS4A8A(+) TAM may serve as a novel cellular target for selective cancer therapy.