Germ-free studies have shown that a lack of microbial growth was associated with increased anti-tumor M1 macrophage differentiation, tumor infiltration with cytotoxic CD8+ and CD4+ T cells, decreased myeloid-derived suppressor cells (MDSCs) and enhanced anti-tumor effects of PD-1 monoclonal antibody therapy [11,38]. Here, CD4 is linked to neoplasm.