Tumor infiltrating lymphocytes are studied for their potential as new clinically useful prognostic and predictive biomarkers in patients with triple negative and HER-2/neu amplified breast cancer. stromal lymphocytic infiltrate called medullary carcinoma has been associated with excellent prognosis after aggressive local therapy in spite of high histologic grade and axillary lymph node metastases [1 2 However over the next several decades a number of studies examined the association between tumor lymphocytic infiltrates and prognosis in more common histologic types of breast cancer and yielded contradictory results [3-6]. The concept of the immune tumor microenvironment’s role in influencing the outcomes of breast cancer patients has resurfaced after the recognition that expression of genes associated with stromal (fibroblast) and immune (macrophage T-cell) components significantly contributes to the global gene expression landscape within the breast tumors as detected by the use of cDNA based gene expression microarrays [7]. High expression of genes typical for lymphocytes (such as CD8) and low expression of genes typical of myeloid cells (such as CD68) was found to indicate better prognosis especially in basal like and HER-2/neu positive breast cancer subtypes [8-10]. This has generated a renewed excitement and multiple investigators began to study the potential association between TILs prognosis and response to systemic therapy for breast cancer. Lymphocytes can be easily detected by analyzing slides of tumor sections stained with hematoxylin and eosin (H&E); a simple and inexpensive technique that can be reliably performed by practically any pathologist. Tumor Infiltrating Lymphocytes and Adjuvant Chemotherapy Trials To study the association between TILs (detected on H&E stain) and survival investigators utilized tumors collected from breast cancer patients who participated in randomized clinical trials of adjuvant chemotherapy. CLU The advantage of this approach is the NSC 23766 availability of samples from large cohorts of patients with abundance of data on clinical characteristics of their disease and their outcomes. By using such an approach Loi and colleagues have demonstrated that a high proportion of TILs (>50%) in NSC 23766 tumors of women with operable triple negative breast cancer enrolled in a large adjuvant chemotherapy NSC 23766 trial called BIG 2-98 was strongly associated with favorable disease free (p=0.014) and overall (p=0.029) survival [11]. In their analysis two distinct subsets of lymphocytes were measured namely (1) stromal TILs (sTILs – mononuclear cells that were present within the tumor stroma but were not in direct contact with invasive carcinoma cells) and (2) intratumoral TILs (iTILs – mononuclear cells that were directly associated with the malignant cells). Their analysis revealed that for every 10% increase in the levels of sTILs there was 17% reduction in the risk of relapse (p=0.025) and death (p=0.023). They also NSC 23766 found close association between higher levels of sTILs and iTILs with infiltrating ductal histology (P<0.001 and 0.48 respectively) high histologic grade (both p<0.001) hormone receptor negativity (both p<0.001) and increased Ki67 expression (>14%; both p<0.001). These results suggest that perhaps lymphocyte predominance could select a subgroup of patients with favorable prognosis despite having other poor clinical and histologic characteristics. No association was found between TIL levels and outcomes of patients with hormone receptor positive or HER-2/neu amplified breast cancer. In the recently presented confirmatory study tumor samples of women with operable triple negative breast cancer from two large adjuvant trials (ECOG 2197 and ECOG 1199) were analyzed for the presence of TILs [12]. Again the investigators found that for every 10% increase in sTILs there was 18% reduction in the risk of distant recurrence and 19% reduction in the risk of death. The presence of iTILs showed a trend towards better outcome but it did not reach statistical significance. On multivariate analysis high levels of sTILs predicted improved disease free distant recurrence free and overall survival independently of other poor clinical and histologic characteristics such as tumor size lymph node metastases or patient age. Similar findings were shown in patients with triple negative and HER-2/neu positive breast cancer enrolled on a phase III randomized FinHER trial that tested different adjuvant chemotherapy regiments with or without trastuzumab. This analysis also suggested for the first time that higher levels of TILs could be associated with increased.