CD4 and neoplasm: Even though there was no strong evidence of correlations, the tumor-infiltrating CD4 and CD8 T cells were characterized by high PD-1 expression in both mice groups (Figure 3), whereas in the PDX mice, the overall immune cell infiltration was low in comparison to the MDA-MB-231 group (PDX: <1%; MDA-MB-231: 32.2% ± 6.1 CD45+).