Carcinoembryonic-Antigen-Related Cell Adhesion Molecule (CEACAM) Expression on Multiple Myelomas Inhibits Their Recognition by Autologous Memory CD8 T Cells

Introduction. Previous studies clearly demonstrated the spontaneous induction and accumulation of functionally competent myeloma antigen–specific memory CD8 T cell responses in the bone marrow of a large proportion of myeloma patients. However, other studies convincingly demonstrated that CD8 T cells from myeloma patients were incabable of lysing autologous myeloma cells. An explanation for this apparent discrepancy is still lacking. CEACAMs are induced on T cells during TCR-specific activation and mediate a rapid blocking of T cell effector functions upon homophilic binding with CEACAMs expressed on target cells. We here addressed the question if myeloma cells might escape recognition by autologous, myeloma-specific CD8 T cells through CEACAM expression.

Methods. Presence of myeloma-specific CD8 T cells was analyzed by IFN-y Elispot assays using separated, bone marrow derived CD8 T cells and myeloma-associated antigen-pulsed autologous DCs or autolgous myeloma cells as antigen presenting cells. Expression of CEACAMs 1–21 was analyzed by differential gene expression profiling of sorted CD138+ myeloma cells from bone marrow of 140 myeloma patients and of respective plasma cells from 14 healthy donors. In addition, the expression of CEACAMs 1, 5, 6 and 8 was analyzed by flowcytometry on the myeloma cell line MM8226 and on CD138+ myeloma cells from altogether 7 myeloma patients. A role of CEACAM on T cell recognition of autologous myeloma cells was analyzed by coculture of CD8 T cells and sorted, autologous myeloma cells in the presence or absence of blocking antibodies against CEACAMs by IFN-γ Elispot assay.

Results. We identified the presence of myeloma-specifcic CD8 T cells in app. 40% of tested myeloma patients (N=20). However, in none of the tested cases T cells were able to directly recognize autologous myeloma cells. Over expression of CEACAM mRNA was found for CEACAM1, 6 and 8 in myeloma cells of up to 20% of patients, but not in plasma cells of healthy donors. Flowcytometric analysis revealed the protein expression of CEACAMs 1 and 6 on 25–50 % of the myeloma cells of all 7 tested patients and on the myeloma cell line MM8226. Blocking of CEACAMs on sorted myeloma cells before their coculture with autologous CD8 T cells resulted in significant T cell responses to myeloma cells in all tested patients (N=6), while in none of these cases the T cells were able to respond to unblocked myeloma cells.

Conclusions. We here demonstrate for the first time the expression of CEACAMs 1 and 6 on freshly isolated myeloma cells. Blocking of these CEACAMs resulted in a spontaneous CD8 T cell response against cocultured autologous myeloma cells which was undetectable in case of unblocked CEACAM expression despite the presence of myeloma-reactive memory T cells. We suggest that CEACAMs on myeloma cells inhibit the re-activation of tumour antigen specific CD8 T cells upon interaction with myeloma cells and may contribute to the well characterized immune resistance of multiple myeloma.