CEACAM5 and neoplasm: To determine if there are increased frequencies of tumor antigen-specific CD8+ T cells, splenocytes were isolated from MC38-CEA tumor-bearing mice on day 17 (Figure 5f) and day 24 (Figure 5g) post-tumor inoculation and co-incubated with p15E, a common MHC class I restricted retroviral protein expressed in the MC38 tumor cell line.