BACKGROUND: CD38, a druggable ectoenzyme, is involved in the generation of adenosine, which is implicated in tumour immune evasion. Its expression and role in prostate tumour-infiltrating immune cells (TIICs) have not been elucidated.

OBJECTIVE: To characterise CD38 expression on prostate cancer (PC) epithelial cells and TIICs, and to associate this expression with clinical outcomes.

DESIGN, SETTING, AND PARTICIPANTS: RNAseq from 159 patients with metastatic castration-resistant prostate cancer (mCRPC) in the International Stand Up To Cancer/Prostate Cancer Foundation (SU2C/PCF) cohort and 171 mCRPC samples taken from 63 patients in the Fred Hutchinson Cancer Research Centre cohort were analysed. CD38 expression was immunohistochemically scored by a validated assay on 51 castration-resistant PC (CRPC) and matching, same-patient castration-sensitive PC (CSPC) biopsies obtained between 2016 and 2018, and was associated with retrospectively collected clinical data. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: mCRPC transcriptomes were analysed for associations between CD38 expression and gene expression signatures. Multiplex immunofluorescence determined CD38 expression in PC biopsies. Differences in CD38 TIIC densities between CSPC and CRPC biopsies were analysed using a negative binomial mixed model. Differences in the proportions of CD38 epithelial cells between non-matched benign prostatic epithelium and PC were compared using Fisher's exact test. Differences in the proportions of biopsies containing CD38 tumour epithelial cells between matched CSPC and CRPC biopsies were compared by McNemar's test. Univariable and multivariable survival analyses were performed using Cox regression models.

RESULTS AND LIMITATIONS: CD38 mRNA expression in mCRPC was most significantly associated with upregulated immune signalling pathways. CD38 mRNA expression was associated with interleukin (IL)-12, IL-23, and IL-27 signalling signatures as well as immunosuppressive adenosine signalling and T cell exhaustion signatures. CD38 protein was frequently expressed on phenotypically diverse TIICs including B cells and myeloid cells, but largely absent from tumour epithelial cells. CD38 TIIC density increased with progression to CRPC and was independently associated with worse overall survival. Future studies are required to dissect TIIC CD38 function.

CONCLUSIONS: CD38 prostate TIICs associate with worse survival and immunosuppressive mechanisms. The role of CD38 in PC progression warrants investigation as insights into its functions may provide rationale for CD38 targeting in lethal PC.

PATIENT SUMMARY: CD38 is expressed on the surface of white blood cells surrounding PC cells. These cells may impact PC growth and treatment resistance. Patients with PC with more CD38-expressing white blood cells are more likely to die earlier.