Program Director/Principal Investigator (Last, First, Middle): Friedberg, Jonathan, W ABSTRACT In addition to the primary effects on calcium homeostasis, Vitamin D has important effects on both innate and adaptive immunity in humans. However, the degree to which these pleotropic effects of Vitamin D influences specific immune responses to infections and cancer in humans is not known. Indolent B-cell lymphoma represents a unique model system to define the effect of Vitamin D on the immune response to cancer. Gene expression profiling studies demonstrate the importance of immune-based signatures in the lymph node microenvironment on prognosis in follicular lymphoma. A recently published study identified that immune-infiltration measured by PD-L2 expression in the microenvironment of follicular lymphoma is strongly predictive of outcome, where low immune infiltration was associated with increased risk of early relapse after initial therapy and ultimately increased risk of death. As Vitamin D can upregulate tissue-specific PD-L1 and PD-L2 surface glycoprotein expression in humans as a direct transcriptional inducer through the Vitamin D receptor, Vitamin D may directly influence the immune response to malignancy. Our current R01 (CA214890) funds a phase 3 clinical trial termed ILyAD (Indolent Lymphoma And Vitamin D), which randomizes patients being treated with rituximab to Vitamin D supplementation with oral cholecalciferol 2000IU daily, or placebo daily. At baseline and in follow-up, all patients have blood obtained for Vitamin D levels, shipped to URMC labs on day of blood draw for central batched analysis using the gold standard liquid chromatography-mass spectrometry (LCMS) method. Therefore, we have the unprecedented opportunity to discover the direct relationship of Vitamin D treatment to immunomodulation in the setting of indolent lymphoma. In this supplement, we propose to assess the tumor biopsies of patients in ILyAD to assess immune infiltration, and correlate to baseline Vitamin D level. To evaluate specific cell populations of interest, we will further characterize the immune infiltration patterns observed in tumors from patients with low and high Vitamin D levels using spatial genomics. Leveraging the power of our randomized, prospective trial, we will determine if Vitamin D supplementation can overcome the negative prognosis of low immune infiltration on the trial. These assays therefore incorporate a potential biomarker of response to Vitamin D, and providing important mechanistic insights around Vitamin D with implications in other benign and malignant immune-reactive conditions. OMB No. 0925-0001/0002 (Rev. 03/2020 Approved Through 02/28/2023) Page 1 Continuation Format Page