[Immunohistochemical study of MSI markers in breast cancer]

To investigate the expression of microsatellite instability (MSI) markers, which is detected by an immunohistochemical technique, and to compare the expression with the PD-L1 status in luminal B, HER2-negative and triple-negative breast cancer.

The investigation included tumors from 40 patients with triple-negative and luminal B, HER2-negative subtypes. Immunohistochemical study was performed using Ventana antibodies: anti-MLH1 (clone M1), anti-MSH2 (clone G219-1129), anti-PMS2 (clone A16-4), and anti-MSH6 (clone SP93). MSI was assessed according to the standard criteria.

The PD-L1-positive status was present in 14 (35%) of the 40 patients. Moreover, MSI-H was detected in only 1 (2.50%) case. The two-year survival rate was 87.5%; it should be noted that the median survival rate was not reached either in the study sample or in the groups divided according to PD-L1 and MSI statuses. The overall survival rate for patients with MSI was 75% (3/4).

The first comparative study of the expression of PD-L1 and immunohistochemical MSI markers, which has been conducted on a small sample, fails to draw unambiguous conclusions, but shows the need to investigate this phenomenon on large samples and by using genetic methods.