Abstract Purpose: Previous studies examining the relationship between smoking and stromal tumor-infiltrating lymphocytes (TILs) in breast cancer (BC) have reported conflicting findings, ranging from no significant association to evidence suggesting that smoking may modify the immune tumor microenvironment (TME) and potentially increase TIL density. This study aimed to clarify whether smoking is associated with TIL density in BC. Methods: Within the prospective Melanoma in Southern Sweden (MISS) study, tissue micro arrays (TMAs) were available for 1,276 women diagnosed with a primary invasive BC between March 1, 1992, and October 31, 2013. The presence of TIL was successfully annotated in 1,173 (91.9%) tumors by a breast pathologist and classified into three categories based on the percentage of stained TILS as low (10%), intermediate (10-49%), or high (≥50%). Associations between TIL categories and ever smoking at study inclusion between 1990-1993, age at BC diagnosis, tumor size, nodal status, histological grade, and triple-negative subtype were assessed using Pearson’s Chi2 test and multivariable linear regression. Two-tailed p-values 0.05 were considered statistically significant. Results: Median age at BC diagnosis was 61 years (interquartile range 53-69 years). Of the TMAs with TILs scored, 839 (71.5%) were classified as low, 259 (22.1%) as intermediate, and 75 (6.4%) as high. Smoking was significantly associated with lower TIL density (p = 0.033, adjusted Ptrend = 0.013). Age at BC diagnosis showed a borderline association with lower TIL density (adjusted Ptrend = 0.060). Higher TIL density was associated with larger tumor size (p 0.001, adjusted Ptrend = 0.023), higher histological grade (p 0.001, adjusted Ptrend 0.001), and triple-negative subtype (p 0.001, adjusted Ptrend 0.001). Nodal positivity was highest in the intermediate group, with no linear association (p = 0.006, adjusted Ptrend = 0.57). Conclusions: In this large prospective cohort, smoking was independently associated with significantly lower TIL density in breast cancer, while associations between TILs and other clinicopathological factors were as expected. These findings suggest that smoking may negatively influence immune TME and shed new light on a mechanism through which smoking could modify response to breast cancer treatments. Further studies are warranted to explore the clinical impact of smoking on TILs in breast cancer. Citation Format: Annelie Marika Augustinsson, Karolin Isaksson, Anikó Kovács, Helena Jernström. Smoking is associated with reduced tumor-infiltrating lymphocyte density in breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2095.
Augustinsson et al. (Fri,) studied this question.