Synchronous bilateral breast cancer (sBBC) occurs after both breasts have been affected by the same germline genetics, reproductive life factors and environmental exposures for decades. It represents an opportunity to decipher the complex interplay between host, tumor, immune system and response to neoadjuvant chemotherapy (NAC). On a cohort of 17575 BCs treated between 2005 and 2012, sBBCs (n=404) were associated with less aggressive proliferative patterns and higher rates of luminal breast cancers (BCs) when compared with unilateral BCs (n=17171). The left and right tumors were concordant for the majority of clinical and pathological features. Tumor pairs of concordant BC subtype were more frequent than pairs of discordant BC subtype, with notably a particularly high frequency of pairs of luminal BCs. Intriguingly, both the levels of tumor infiltrating lymphocytes (TILs) and the response to NAC were modified by the subtype of the contralateral tumors. Whole exome sequencing and RNAseq analyses revealed that left and right tumors were independent from a somatic mutation and transcriptomic point of view, while primary tumors (PT) before NAC and specimens with residual disease (RD) after NAC were more closely related. The analysis of the TCR repertoire identified very little overlap between patients, while common clones were shared in bilateral tumors within each patient. After NAC, the TCR repertoire of RD was enriched and expanded with clones edited by the contralateral PT.
