## 3 Results
### GPRC5A Is Significantly Associated with an Inflammatory Immune Environment  
The immune composition of the GBM microenvironment is crucial for tumor growth and therapy resistance. We investigated the relationship between GPRC5A expression and immune infiltration in GBM using immune scores and ESTIMATE scores derived from the ESTIMATE method. Our analysis revealed a positive association between GPRC5A expression and both immune and ESTIMATE scores (Figure 4A). Figure 4: The association between GPRC5A and the immune microenvironment in GBM. (**A**) Differential correlation of immune score and estimate scores correlate with GPRC5A log2 (norm count + 1). All statistical correlations were analyzed using the Spearman test (n = 166). (**B**) Top 10 canonical immune cell gene sets from GSEA for high GPRC5A expression versus low expression with a p-value shown with code color. (**C**) Spearman correlation bubble plot of various immune cells associated with GPRC5A expression. The significance shows the correlation coefficient with a \(-\)log10 \(p\)-value. (**D**) Heatmap displaying the distribution of immune cells across GBM subtypes compared to GPRC5A. Statistical testing was performed with the Spearman correlation test, and the significant \(p\) value is displayed on the heatmap. \({}^{***}p<0.0001\), \({}^{**}p<0.001\), \({}^{**}p<0.01\), and \({}^{*}p<0.05\). To further explore the immune-related pathways associated with GPRC5A, we evaluated immune-related pathways by comparing the high to low GPRC5A expression groups. We observed a prominent enrichment of the gene set related to the interaction between lymphoid and non-lymphoid cells, with various inflammatory immune cells, including natural killer (NK) cells, B cells, CD4 T helper cells, natural killer T (NKT) cells, monocytes, CD8 T cells, macrophages, neutrophils, and granulocytes (Figure 4B). To further distinguish the myeloid populations in the brain, specifically the differences between macrophages and microglia, we conducted a correlation analysis using macrophage markers (_CD68_, _CD163_, and _CD209_) and microglia markers (_TMEM119_, _CX3 CR1_, and _P2 RY12_). Our analysis revealed a positive correlation with macrophage markers, but not with microglia markers (Appendix A Table A1). We analyzed tumor-immune interactions using the TISIDB web portal to evaluate the immune subtypes linked to GPRC5A (Figure 4C). This analysis showed a positive correlation between GPRC5A and CD4 T-reg and MDSCs, known to be key players in the maintenance of an immunosuppressive microenvironment of GBM tumors. Among CD4 T cells, GPRC5A was positively correlated with inflammatory subtypes, such as Th1 and Th17, while showing a negative association with Th2 cells. Although CD8 T cell pathways were enriched (Figure 4B), no significant correlation with GPRC5A was observed. Regarding B cells, only memory B cells displayed a positive correlation with GPRC5A, whereas activated B cells (actB) exhibited a negative association. Immature or naive B cells did not show any significant correlation. Among NK cell subsets, only cytotoxic NK cells (NK dim) were positively correlated with GPRC5A, while NK bright cells, known for their weaker cytotoxicity, did not show any significant correlation. For granulocytes, only mast cells were positively associated with GPRC5A, whereas eosinophils showed no significant correlation. Finally, other myeloid cells, such as monocytes, and macrophages, exhibited a positive correlation with GPRC5A, consistent with the pathway enrichment analysis (Figure 4C). Our immunoprofiling also confirmed a correlation with dendritic cells, such as plasmacytoid DC (pDCs), and phenotypes of immature and activated DC. To evaluate the immune landscape associated with GPRC5A, we compared immune cell populations across the four GBM subtypes in relation to GPRC5A expression. Our analysis revealed that GPRC5A shares a similar immune profile with the mesenchymal subtype, while the proneural subtype exhibited an inverse correlation with the GPRC5A immune landscape (Figure 4D).