Effective antitumor activity of immune checkpoint inhibitors targeting the PD-(L)1 axis depends on intact interferon-γ (IFN-γ) signaling, as disruption of this pathway eliminates therapeutic benefit. While epithelial cells generally lack baseline expression of major histocompatibility complex (MHC) class II molecules, exposure to IFN-γ can induce their expression on tumor cells. This acquired tumor cell MHC class II expression has been proposed as a functional indicator of immune engagement and sensitivity to PD-(L)1 blockade. Retrospective clinical analyses across multiple malignancies have linked tumor-specific MHC class II (tsMHC-II) expression with favorable outcomes following immune checkpoint therapy. The ANICCA-Class II trial was designed to prospectively determine whether tsMHC-II expression could identify patients with proficient mismatch repair colorectal cancer (pMMR CRC) who may benefit from PD-1 inhibition. In addition, we examined whether the immunoscore–immune checkpoint (IS-IC), previously suggested as a predictive tool, could stratify outcomes in this setting. Patients with confirmed locally advanced or metastatic pMMR CRC, an ECOG performance status of 0–2, and tumor MHC class II expression greater than 1% were enrolled. Eligible participants were adults aged 18 years or older. Nivolumab was administered at a fixed dose of 480 mg every four weeks for up to 24 cycles. The primary endpoint was durable clinical benefit, defined as freedom from disease progression at the third protocol-defined radiologic assessment, approximately 27 weeks after therapy initiation. Secondary endpoints included progression-free survival and overall survival. Thirty-five patients received study treatment, with nearly two-thirds demonstrating tsMHC-II expression of at least 5%. Durable clinical benefit was achieved in three patients, yielding an observed rate of 8.6%. Bayesian modeling estimated the true durable benefit rate at 11%, with a 95% credible interval ranging from 3% to 22%, and demonstrated an extremely low likelihood that the true rate exceeded 30%. Increasing the tsMHC-II expression threshold did not enhance predictive performance for disease control. All patients who experienced durable benefit lacked liver metastases, corresponding to a benefit rate of 23.1% in patients without hepatic involvement and no observed benefit among those with liver metastases. Survival analyses showed significantly longer progression-free and overall survival in patients without liver metastases. No association was identified between high IS-IC status and treatment duration or tumor growth suppression. Prospective selection based on tumor-specific MHC class II expression does not identify patients with metastatic pMMR colorectal cancer who derive meaningful benefit from PD-1 monotherapy. Similarly, our findings do not support a predictive role for the immunoscore–immune checkpoint in this context, although the analysis is limited by sample size. The consistently poor outcomes observed in patients with liver metastases underscore the dominant immunosuppressive influence of hepatic involvement and highlight the need for therapeutic strategies capable of reversing liver-driven systemic immune tolerance.
