Background: The density of tumour-infiltrating lymphocytes (TIL) has been proposed as an independent predictor of outcome in patients with colorectal cancer. However, the relative roles of TIL density, nodal status, and microsatellite instability (MSI) in predicting tumour progression to metachronous metastasis remain to be elucidated. The aim of this study was to assess the relationship between the density of CD3+ TIL and the postsurgical occurrence of distant-organ metastases in a large series of patients with deeply invading and MSI-typed colorectal cancer. Methods: Per cent areas of immunoreactivity due to CD3+ TIL at the invasive margin of the tumour (CD3+ TILIM) were measured by computer-assisted image analysis in 286 tissue specimens from pT3 or pT4 MSI-tested colorectal cancer. Tissue samples were taken from consecutive patients who underwent resection at the IRCCS Istituto Clinico Humanitas, Rozzano, Milan, Italy, from January, 1997, to November, 2004, for colorectal cancer with no evidence of metastasis at diagnosis. Occurrence of metachronous metastasis, disease-specific survival (DSS), and disease-free survival (DFS), were assessed retrospectively in relation to per cent immunoreactivity. Findings: CD3+ TILIM density was higher in MSI colorectal cancer than in mismatch repair-system-proficient tumours (6·53% vs 2·19%; pIM densities, colonic site, and absence of nodal involvement were significantly associated with a lower risk of metachronous metastasis, but only the interaction between CD3+ TILIM density and N-stage was significant on multivariate analysis (p=0·002). On separate analysis of node-negative colorectal cancer, increasing percentage of CD3+ immunoreactive area progressively reduced the risk of metachronous metastasis (5%, 0·06, 0·01-0·48, p=0·008). Conversely, no significant association was seen between CD3+ immunoreactive area and risk of metachronous metastasis in node-positive colorectal cancer. Accordingly, CD3+ TILIM density was associated with a better DSS (p=0·01) and DFS (p=0·006) only in patients with node-negative colorectal cancer. In primary tumours that had progressed to metachronous metastasis, stage III tumours had higher CD3+ TILIM densities than stage II tumours (p=0·0004). Interpretation: Metachronous metastases are unlikely to arise from node-negative colorectal cancers with a high-density CD3+ TILIM, whereas high densities of CD3+ TILIM are not associated with the absence of postsurgical metastasis in patients with node-positive colorectal cancer. Our data suggest that densities of CD3+ TILIM cannot be used as an independent predictor of clinical outcome in patients with stage III colorectal cancer and, at least for now, the tumour-node-metastasis classification should remain the preferred prognostic system. Our findings are consistent with a relationship between nodal involvement and tumour immunoevasion. Funding: MIUR (Ministero dell'Istruzione, dell'Università e della Ricerca), Target Project Oncologia 2006, and Alleanza Contro il Cancro.
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