A link between TGFbeta and intraepithelial tumor infiltrating lymphocytes in microsatellite instability-high colorectal cancer

Microsatellite instability high (MSI-H) colorectal cancers (CRCs) possess such features as heavy infiltration by intraepithelial tumor infiltrating lymphocytes (iTILs), a relatively good prognosis, and truncating mutations in the type II TGFbeta receptor (TGFbetaRII). The aim of this study was to examine the possibility of a connection between these phenomena in MSI-H CRCs. Immunohistochemical analysis identified increased tumor TGFbeta expression as predictive of high iTIL counts in MSI-H CRCs but not in microsatellite stable (MSS) CRCs. Abundant iTILs were associated with longer survival times only for patients with MSS tumors. These results indicate that iTILs in MSI-H CRCs are not part of an active anti-tumor immune response. Since TGFbeta can autoinduce itself, the extent of sensitivity of TGFbetaRII deficient MSI-H CRC cell lines was determined using an inhibitor of the complementary TGFbeta type I receptor. MSI-H CRCs remained sensitive to endogenously produced TGFbeta via this receptor. However, autoinduction of TGFbeta by endogenous cytokine was not seen and seems unlikely to explain the association between high TGFbeta and iTILs. The search for an alternate signaling mechanism to account for persistent endogenous TGFbeta sensitivity led to the identification of a membrane-bound form of TGFbeta on CRC cells. This membranous-TGFbeta was able to initiate cell-cell contact dependent signaling in both adjacent CRC cells and lymphocytes, thereby representing a novel mode of paracrine tumor TGFbeta signaling. In order to explore the origin and effects of TGFbeta on CRC/iTIL interactions, a Transwell system was used allowing for basolateral migration of either peripherally- or intraepithelially-derived lymphocytes into CRC monolayers. MSI-H CRCs preferentially retained and expanded lymphocytes from the latter population and were found to induce a lesser degree of T cell activation than their MSS counterparts. TGFbeta played a modulatory, though not definitive, role in the observed CRC/lymphocyte interactions. Inferior activation by MSI-H CRCs may explain the lack of prognostic benefit conferred by iTILs within these cancers. We have thus documented within MSI-H CRCs an association between iTILs and TGFbeta which is not mediated by TGFbetaRII mutation, a lack of association between iTILs and survival, and a probable origin for iTILs within a pretolerized intraepithelial lymphocyte subset.