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Background Due to its rarity and histological diversity, the tumor microenvironment (TME) of appendiceal cancer (AC) remains understudied. In colorectal cancer (CRC), B cells have demonstrated context-dependent roles, contributing to either tumor progression or suppression. This study aims to evaluate subset B cell density in AC and its association with clinicopathological features, previously analyzed immune cell populations and patient outcomes.Methods Formalin-fixed, paraffin-embedded (FFPE) AC tissue from 158 patients was collected and assembled into tissue microarrays (TMAs) for high-throughput analysis. TMAs were stained for CD20 + and CD138+ to assess overall B lymphocyte and plasma cell densities, respectively. Tumor and immediate adjacent (<3mm) peritumoral tissue were included. Quantitative digital imaging (QuPath) was utilized to calculate positive cell density per unit tissue volume. CD20+ and CD138+ cell densities were then correlated with patient factors (age, sex), tumor characteristics (histologic subtype, grade, stage, nodal status, disease extent, treatment history), previously quantified immune markers (CD3+, CD8+, CD86+/CD68+, CD206+/CD68+), and oncologic outcome (progression-free/PFS or overall/OS survival). Continuous variables were tested for association using Spearman’s rank correlation test; categorical variables using the Kruskal-Wallis test; and survival outcomes using Cox regression.Results Primary AC tumors contained higher densities of overall B cells (CD20+) and plasma cells (CD138+; figure 1), relative to peritoneal metastases (p<0.01). This is consistent with the high density of lymphoid follicles and germinal centers found in the normal anatomic appendix. Neither marker correlated with patient age, sex, or tumor characteristics (histologic subset, grade or stage). Plasma cell density was inversely associated with carcinoembryonic antigen (CEA) levels (r=-0.2, p=0.02) and positively associated with disease volume as indicated by peritoneal carcinomatosis index (PCI; r=0.2, p=0.03). B cell density was associated with T cell density within AC tissues (r=0.3, p=0.02); no other associations were observed among other analyzed cell populations. Finally, we detected a low-CD20 subset of tumors (lower tertile) that were associated with poor progression-free (hazard ratio/HR 1.89 [95%CI 1.06 ,3.3], p=0.03) and overall (HR 4.17 [95%CI 0.98, 16.7], p=0.04; figure 1).Conclusions B cell density varied widely across AC tissue samples, with higher densities in primary tumors versus peritoneal metastases. Although few correlations were found between B cell/plasma cell density and conventional patient, tumor and treatment characteristics, low B-cell density was a significant prognosticator of poor progression and overall survival. This association will warrant further study in the future, with larger patient populations and a more refined B cell subset analysis.Acknowledgements The funds for this study were provided by ACPMP.Ethics Approval This retrospective study was carried out in accordance with the principles of the Declaration of Helsinki and was approved by the Institutional Review Board of the Allegheny Health Network (AHN) Cancer Institute (protocols 2021-255-AHNMR, 2020-258-AGH and 2022-116-AHNCI-AGH). Waiver of informed consent was granted in accordance with 2018 Common Rule (45 CFR 46), exemption category 4. All patient records and biological samples were de-identified prior to analysis to ensure confidentiality and adherence to ethical standards, and patients were not contacted during the study.Abstract 752 Figure 1Representative slide from immunohistochemical staining. CD20 and CD138 positive densities compared to primary vs. metastatic site. Progression-free and overall survival for CD20 densities