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

No blockade of CEACAM1-CEACAM1 interaction by CC1 was observed in either protein or cell-based ELISA ( Figure 2D , 2E). In both assays we unexpectedly observed an increasing CEACAM1-CEACAM1 binding signal with increasing concentrations of CC1 ( Figure 2D and 2E). Similar results were obtained by flow cytometry (data not shown) when soluble mCEACAM1 binding to mCEACAM1 transfected cells was analyzed in the presence of CC1 antibody. These data suggested that CC1 antibody may provide a bridging effect by linking two CEACAM1 molecules together rather than blocking the homophilic interaction of CEACAM1. For the bridging effect to occur, we hypothesized that a two-arm full-length IgG molecule would be needed. To address this question, we also generated a Fab fragment of CC1. When the Fab of CC1 was used in the same assay, we did not observe the increasing signal with increasing concentration of Fab. Instead, the Fab of CC1 blocked the CC1 homophilic interaction in cELISA ( Figure 2F ) and in protein ELISA (data not shown). Because of the undetectable binding of TIM-3 to CEACAM1 in our assays, the potential effect of CC1 on this interaction was not studied.