Background: While immune checkpoint inhibitors (ICI) have improved outcomes in patients (pts) with metastatic renal cell carcinoma (mRCC), acquired resistance (ARX) is commonly encountered. We aimed to characterize the genomic and transcriptomic correlates underlying ARX in ICI-treated mRCC patients. Methods: We identified pts with mRCC at Dana-Farber Cancer Institute and Fox Chase Cancer Center, treated with ICI-based regimens who had an initial response to therapy followed by disease progression. Tissue samples from the primary tumors or metastases were collected before ICI treatment and after ARX from a treatment-emergent escape lesion. Whole exome sequencing (WES) and bulk RNA-sequencing (RNA-seq) were performed. Somatic mutations were called using the Cancer Genome Analysis pipeline. Differential gene expression analysis (DGE) was run using DESeq2, followed by gene set enrichment analysis (GSEA). Immune cell fractions were estimated using CIBERSORTx. Cell fraction and tertiary lymphoid structures (TLS) scores were compared between post and pre-ARX samples using the Wilcoxon test. Results: A total of 41 samples (n=23 pre-ARX and n=18 post-ARX) were collected from 14 unique patients. After quality control, WES and RNA-seq data were available for 27 samples (14 pre-ARX and 13 post-ARX) and 9 samples (3 pre-ARX and 6 post-ARX), respectively. No discernable enrichment of gene mutations was detected in the post-ARX setting compared to the pre-ARX. DGE identified 61 downregulated immunoglobulin genes in the post-ARX setting. GSEA revealed significant depletion of pathways associated with B-cell and T-cell function in post-ARX samples (all adjusted p<0.05). Similarly, a trend towards decreased naïve B cell and CD8+ T cell fractions was observed in the post-ARX setting (p=0.17 for both). Three previously reported TLS-associated expression signatures were depleted in the post-ARX samples compared to pre-ARX (p=0.015, p=0.065 and p=0.065, respectively). Immunohistochemistry assessment of 3 samples from one pt using ant-CD3 and CD20 antibodies confirmed the presence of TLS in the pre-ARX sample and their absence in two different post-ARX samples. Conclusions: In our cohort of pts with mRCC, no gene mutations were implicated as drivers of ARX. However, ARX was associated with downregulation of B-cell signaling and humoral mediated immunity, as well as absence of TLS. Ongoing efforts are in progress to include more samples.