β2-microglobulin expression is associated with aggressive histology, activated tumor immune milieu, and outcome in colon carcinoma.

Objectives: We sought to assess the expression of human leukocyte antigen (HLA) proteins and β2-microglobulin (B2M) in tumor cells and the relationship with immune microenvironment and outcome in colorectal cancer (CRC).

Methods: A total of 953 CRC cases were evaluated by immunohistochemistry for HLA class I, HLA class II, and B2M. The expression level of these biomarkers was correlated with clinicopathologic information, BRAF V600E and mismatch repair (MMR) proteins, and the quantitated expression levels of immune cells (CD8 and CD163) and immune regulatory proteins (FoxP3, programmed cell death 1 ligand 1 [PD-L1], and LAG3).

Results: We found that B2M-low tumors were statistically correlated with aggressive histologic features, including higher stage, higher grade, extramural venous invasion, perineural invasion, and distant metastasis. Expression of B2M was positively correlated (R2 = 0.3) and significantly associated with MMR-deficient tumors (P < .001); B2M-low tumors were also associated with an "immune cold"' microenvironment, including a reduced number of immune cells (CD8 and CD163), reduced expression of immune regulatory proteins by immune cells (PD-L1, FoxP3, and LAG3), and reduced tumor cell expression of PD-L1. These B2M-low tumors correlated with lower disease-specific survival (P = .018), a finding that maintained significance only for the proficient MMR cohort (P = .037).

Conclusions: Our findings suggest that B2M expression may support predictive models for both outcome and checkpoint inhibitor therapy treatment response for colorectal adenocarcinoma.