FIGURE 6 from Siglec-15 Promotes Evasion of Adaptive Immunity in B-cell Acute Lymphoblastic Leukemia

Sig15 promotes an immunosuppressive tumor microenvironment in B-ALL. A–M, Unirradiated WT C57BL6 mice were injected via tail vein with 5 × 10⁵ Cas9-only (n = 8 from two independent experiments) or Sig15 KO (n = 8) leukemia. Bone marrow was harvested 7 days later for highly dimensional flow cytometry of non-leukemia bone marrow populations. Healthy, leukemia-naïve mice (Naive) were included as controls (n = 6). CD3⁺ T cells (A), NK cells (B), classical dendritic cells (cDC; C), and neutrophils (D) were variably present at higher totals in the bone marrow of Sig15 KO recipient mice compared with control leukemia. E, Representative contour plots of T-cell populations. Proportions of CD8⁺ (F) but not CD4⁺ (G) T cells were higher in Sig15 KO recipients than control leukemia, increasing the CD8⁺/CD4⁺ T-cell ratio (H) to levels comparable with Naive mice. I, Degranulated CD8⁺ T cells were higher in Sig15 KO recipient mice, as were SLEC (J) and MPEC (K) CD8⁺ T-cell populations. Early memory-like populations consistent with central (L) and effector memory (M) CD8⁺ T-cell populations were also higher in Sig15 KO recipients. N–P, From the same experiments, bone marrow supernatant from control and Sig15 KO leukemia recipient mice was analyzed via 44-plex cytokine/chemokine assay. Levels of IL6 (N), LIF (O), and IL5 (P) were all significantly lower in the bone marrow of Sig15 KO recipients as compared with control leukemia (*, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001). Sig15 expression on B-ALL cells suppresses CD8⁺ T-cell expansion and activation and contributes overall to formation of a more proleukemia bone marrow microenvironment.