Sialic acid-binding immunoglobulin-like lectin-15 expression on peritumoral macrophages is a favorable prognostic factor for primary central nervous system lymphoma patients

Abstract

Sialic acid-binding immunoglobulin-like lectin-15 (Siglec-15) is a new immune checkpoint molecule and its role of primary central nervous system lymphoma (PCNSL) tumor microenvironment has been unclear. We explored the Siglec-15 and programed death-ligand 1 (PD-L1) expression in tumor tissues and analyzed the association between the expression of these molecules and overall survival in newly diagnosed PCNSL. A total of 60 patients diagnosed with diffuse large B-cell lymphoma in PCNSL were included in this study. The Siglec-15 and PD-L1 expression on tumor cells, intratumoral macrophages and peritumoral macrophages were immunohistochemically evaluated. The expression of Siglec-15 and PD-L1 was greater in macrophages than in tumor cells. Regarding peritumoral macrophages, the number of Siglec-15-positive samples (n = 24) was greater than the number of PD-L1-positive samples (n = 16). A multivariate Cox analysis showed that the Siglec-15 positivity of peritumoral macrophages and performance of high-dose methotrexate-based chemotherapy were independent predictors of overall survival (hazard ratio: 0.295 and 0.322, respectively). The Kaplan-Meier survival curves showed that patients with Siglec-15-positive peritumoral macrophages had longer overall survival than those with Siglec-15-negative peritumoral macrophages (median overall survival: 3018 days and 746 days, respectively; p = 0.0290). Our findings indicate that the expression of Siglec-15 on peritumoral macrophages induces a favorable outcome in PCNSL patients.

Figure 1

The expression of Siglec-15 (A positive tumor cells, B positive intratumoral macrophages, C positive peritumoral macrophages) and the expression of PD-L1 (D positive tumor cells, E positive intratumoral macrophages, F positive peritumoral macrophages) in tumor tissue (A,B,D,E magnification × 400, and C,F magnification × 200). Dark red-brown color represented the presence of antigens and antigen-positive cells indicated with arrows in the figures.

Figure 2

The correlation of the Siglec-15 and PD-L1 expression in our cohort (A tumor cells, B intratumoral macrophages, C peritumoral macrophages). In the majority of the patients (37 samples) tumor cells were negative for Siglec-15 and PD-L1. Larger numbers of samples had Siglec-15- and PD-L1-positive macrophages in comparison to tumor cells. With regard to the expression on peritumoral macrophages, the number of Siglec-15-positive samples (n = 24) was greater than the number PD-L1-positive samples (n = 16).

Figure 3

The histopathological findings in the tumoral and peritumoral tissue. Hematoxylin and eosin staining showed the high cellularity, diffuse growth, and perivascular spread of the tumor cells (A). The tumor cells were positive for CD20 (B). CD68 staining revealed the existence of intratumoral macrophages and peritumoral macrophages (C). The macrophages were positive for Siglec-15 while the tumor cells were negative for Siglec-15 (D). (magnification × 100). Dark red-brown color represented the presence of antigens in (B–D).

Figure 4

Double-immunofluorescent staining of CD68 and Siglec-15. CD68 staining (A green). Siglec-15 staining (B red). DAPI staining (C blue). Merged images (D). Representative images of PCNSL peritumoral tissue with the co-expression of CD68 and Siglec-15. (magnification × 1000).

Figure 5

Kaplan–Meier survival curves for PCNSL patients stratified by whether or not peritumoral macrophages expressed Siglec-15. CI confidence interval, NA not analyzed.