Ablation of B7-H3 but Not B7-H4 Results in Highly Increased Tumor Burden in a Murine Model of Spontaneous Prostate Cancer

Abstract

The costimulatory molecules B7-H3 and B7-H4 are overexpressed in a variety of human tumors and have been hypothesized as possible biomarkers and immunotherapeutic targets. Despite this potential, the predominating uncertainty about their functional implication in tumor-host interaction hampers their evaluation as a target for cancer therapy. By means of a highly physiologic, spontaneous tumor model in mice, we establish a causal link between B7-H3 and host tumor control and found B7-H4 to be redundant.

Figure 1. Greatly increased growth of murine prostate tumors in the absence of B7-H3

(a) B7-H3 and (b) B7-H4 mRNA expression in the DL prostate lobes of male TRAMP⁺ or age-matched TRAMP⁻ littermate control mice at indicated time points measured by qPCR and calculated by 2^{(-ΔΔ Ct)} ± SEM . (n≥3 per group) (c) Representative images of UG of 18-week and (d) 30-week old TRAMP⁺wt, TRAMP⁺B7-H3^−/− and age-matched TRAMP⁻wt or TRAMP⁻B7-H3^−/− littermate control mice, respectively. (e) Masses of UG and DL prostates of TRAMP⁺wt, TRAMP⁺B7-H3^−/−, TRAMP⁺B7-H4^−/− and corresponding TRAMP⁻wt, TRAMP⁻B7-H3^−/− or TRAMP⁻B7-H4^−/− littermate control mice, respectively, evaluated at 12 weeks, (f) 18 weeks and (g) 30 weeks of age and normalized to whole body weight ±SEM. (TRAMP⁺wt: n=39, TRAMP⁺B7-H3^−/−: n=48, TRAMP⁺B7-H4^−/−: n=21, TRAMP⁻wt: n=33, TRAMP⁻B7-H3^−/−: n=28, TRAMP⁻B7-H4^−/−: n=14) Unpaired t test. * p< 0.05, ** p< 0.01, *** p< 0.001.

Figure 2. Lack of B7-H3 expression on radio-resistant tissue accounts for elevated tumor growth and accumulation of regulatory FoxP3⁺ T cells

Percentages of tumor-infiltrating (a) FoxP3⁺ cells and (b) Ki67⁺FoxP3⁺ cells, (c) total FoxP3⁺ cell-counts per mg of tumor, and (d) ratio of FoxP3⁺ to CD8⁺ T cells isolated from TRAMP⁺wt and TRAMP⁺B7-H3^−/− tumors at 18 weeks of age and analyzed by flow cytometry ± SEM (n≥5). Masses of (e) UG and (f) DL prostates of indicated bone marrow chimeric TRAMP⁺wt, TRAMP⁺B7-H3^−/− and corresponding TRAMP⁻ control mice evaluated at 30 weeks of age and normalized to whole body weight ±SEM (TRAMP⁺: n≥6). Percentages of tumor-infiltrating (g) FoxP3⁺ cells and (f) Ki67⁺FoxP3⁺ cells isolated from indicated bone marrow chimeric TRAMP⁺ mice at 30 weeks of age. Unpaired t test. * p< 0.05, ** p< 0.01, *** p< 0.001, **** p<0.0001