Characterization of murine CEACAM1 in vivo reveals low expression on CD8(+) T cells and no tumor growth modulating activity by anti-CEACAM1 mAb CC1

McLeod, Robbie L.; Angagaw, Minilik H.; Baral, Toya Nath; Liu, Liming; Moniz, Raymond Joseph; Laskey, Jason; Hsieh, SuChun; Lee, Mike; Han, Jin-Hwan; Issafras, Hassan; Javaid, Sarah; Loboda, Andrey; Sadekova, Svetlana; O'Connor, Joann A.; Tse, Archie; Punnonen, Juha

We found that CEACAM1 was expressed at relatively high levels on freshly harvested tumor infiltrating B, NK cells, and MDSCs, although the proportion of these cells was low. Additionally, the present results demonstrated low expression of CEACAM1 on CD8+ tumor infiltrating T cells across three different tumor models. Our findings diverge from previously published observations in which CEACAM1 was identified as a marker of exhausted T cells in the tumor microenvironment with majority of the CD8+ T cells in human melanoma samples expressing CEACAM1 by IHC [20] . In mice, a significant fraction of tumor infiltrating CD8+ T cells have also been reported to express CEACAM1 [21] . The reason for apparent differences in tumor CEACAM1 expression profiles between our study and others is unclear. Differences in the tumor microenvironments or staining conditions cannot be ruled out, but this appears unlikely Figure 5 : The effect of CC1 in alone and combination with the anti-PD-1 antibody (muDX400) in subcutaneous MBT2 (bladder) and A20 (B cell lymphoma) syngeneic mouse models. CC1 (30 mg/kg) alone did not alter tumor growth rates compared to controls. muDX400 (5 mg/kg) and muDX400 plus CC1 (30 mg/kg) significantly slowed tumor growth. P < 0.05 (Kruskal-Wallis in conjunction with Mann-Whitney U post hoc analysis) compared to control at day 14. due to strong staining of CC1 observed on myeloid cells and CT26 tumor cells used by both us and others [21] . Immune checkpoint inhibitors, such as anti-PD-1 mAbs, have demonstrated remarkable clinical benefits in multiple cancer types [27] [28] [29] . Given that CEACAM1 expression on both immune cells and tumor cells is reported to be upregulated following activation by IL-2 or tumor cell contact [30, 31] , we also investigated whether anti-PD-1 treatment enhances CEACAM1 expression on CD8+ T cells. CEACAM1 expression on tumor infiltrating T cells remained low whether or not the animals were treated with anti-PD-1 (muDX400) over multiple doses at levels that provided strong anti-tumor activity in vivo. These data suggest that CEACAM1 has a low impact as a direct regulator of tumor infiltrating T cells.