Background: Primary breast cancer (BC) has shown a higher immune infiltration than the metastatic disease, justifying the optimal scenario for immunotherapy. Recently, neoadjuvant chemotherapy (NAC) combined with immune checkpoint inhibitors has demonstrated a gain in pathological complete responses (tpCR) in patients with BC. The aim of our study is to evaluate the safety, feasibility, and efficacy of the addition of dendritic cell vaccines (DCV) to NAC in HER2-negative BC patients. Methods: Thirty-nine patients with early BC received DCV together with NAC conforming the vaccinated group (VG) and compared with 44 patients as the control group (CG). All patients received anthracyclines and taxanes-based NAC (ddECx4→Dx4) followed by surgery ± radiotherapy ± hormonotherapy. Results: The tpCR rate was 28.9% in the VG and 9.09% in the CG ( p = 0.03). Pathological CR in the triple negative (TN) BC were 50.0% versus 30.7% ( p = 0.25), 16.6% versus 0% in luminal B ( p = 0.15), and none among luminal A patients in VG versus CG, respectively. Impact of DCV was significantly higher in the programmed cell death ligand 1 (PD-L1) negative population ( p < 0.001). PD-L1 expression was increased in patients with residual disease in the VG as compared with the CG ( p < 0.01). No grade ⩾3 vaccine-related adverse events occurred. With a median follow-up of 8 years, no changes were seen in event-free survival or overall survival. Phenotypic changes post DCV in peripheral blood were observed in myeloid-derived suppressor cells (MDSC), NK, and T cells. Increase in blood cell proliferation and interferon (IFN)-γ production was detected in 69% and 74% in the VG, respectively. Humoral response was also found. Clonality changes in TCR-β repertoire were detected in 67% of the patients with a drop in diversity index after treatment. Conclusion: The combination of DCV plus NAC is safe and increases tpCR, with a significant benefit among PD-L1-negative tumors. DCV modify tumor milieu and perform cellular and humoral responses in peripheral blood with no impact in outcome. Trial registration: ClinicalTrials.gov number: NCT01431196. EudraCT 2009-017402-36.

Background: The addition of dendritic cell vaccines (DCV) to NAC could induce immune responses in those patients with residual disease (RD) by transforming the tumor microenvironment. Methods: Core diagnostic biopsies and surgical specimens from 80 patients (38 in the vaccinated group plus NAC (VG) and 42 in the control group (CG, treated only with NAC) were selected. We quantify TILs (CD8, CD4 and CD45RO) using immunohistochemistry and the automated cellular imaging system (ACIS III) in paired samples. Results: A CD8 rise in TNBC samples was observed after NAC plus DCV, changing from 4.48% in the biopsy to 6.70% in the surgical specimen, not reaching statistically significant differences (p = 0.11). This enrichment was seen in up to 67% of TNBC patients in the experimental arm as compared with the CG (20%). An association between CD8 TILs before NAC (4% cut-off point) and pathological complete response in the VG was found in the univariate and multivariate analysis (OR = 1.41, IC95% 1.05–1.90; p = 0.02, and OR = 2.0, IC95% 1.05–3.9; p = 0.03, respectively). Conclusion: Our findings suggest that patients with TNBC could benefit from the stimulation of the antitumor immune system by using DCV together with NAC.

Purpose: The addition of dendritic cell vaccines (DCV) to neoadjuvant chemotherapy (NAC) could induce immune biomarker changes in those patients with residual disease (RD) by transforming tumor microenvironment. Methods: Core diagnostic biopsies and surgical specimens from 80 patients (38 in the Vaccinated Group plus NAC (VG) and 42 in the Control Group (CG) treated only with NAC) were selected. We quantify TILs (CD8, CD4 and CD45RO) using Immunohistochemistry (IHC) and the Automated Cellular Imaging System (ACIS III) in the core-diagnostic biopsies and in the surgical specimens, to compare the amount of TILs in each group. Results: A CD8 rise in TNBC samples was observed after NAC plus DCV, changing from 4.48% in the biopsy to 6.70% in the surgical specimen, not reaching statistically significant differences (p = 0.11). TNBC patients in the CG showed a TILs drop from 2.71% in the biopsy to 0.18% in the surgical specimen (p = 0.5). We also found that 66.7% (4/6) of TNBC patients from VG registered an increase in TILs after treatment as compared to 20% (1/5) of TNBC patients in the CG (p=0.24). This phenomenon is not observed in the other biologic subtypes. An association between before NAC CD8 TILs (4% cut-off point ) and pathological complete response in the VG was found in univariate and multivariate analysis (OR=1.41, IC95% 1.05-1.90; p=0.02, and OR=2.0, IC95% 1.05-3.9; p=0.03, respectively).Conclusion: Our findings suggest that patients with TNBC especially benefit from the stimulation of the antitumor immune system by using DCV pulsed with tumor antigens.

585 Background: Breast cancer (BC) in early stages exhibit a naïve and competent immune system that translates into a more prominent TIL infiltration and higher PD-L1 expression as compared to the advanced BC scenario were immunoescape and exhaustion are more prevalent. Expression of PD-L1 has been related to a better pCR when immune checkpoint inhibitors (IPI) have been added to neaodjuvant chemotherapy (NACT) in triple negative BC (TNBC). Our prior results shown dendritic cells vaccines (DCV) increased pCR in both TNBC and luminal B subtypes, with an absolute gain of 20% (p = 0.03) and a safe tolerance. Methods: Eighty-three HER2 negative BC patients with untreated stage II-III were included: 39 patients from the NCT01431196 trial that combine NACT with autologous DCV and 44 patients from a historic control group treated with the same NACT alone. NACT consists of dose dense Epirubicin plus Cyclophosphamide for 4 cycles sequenced to Docetaxel for 4 cycles. PD-L1 expression was measured in the membrane of tumoral cells with monoclonal rabbit anti PD-L1 28.8 pharmaDX (DAKO, Agilent Technologies) in FFPE samples at diagnosis. Primary endpoint was pathologic complete response (pCR) stratified by PD-L1 expression (positive or negative), while secondary endpoints were event-free survival (EFS) and overall survival (OS), also stratified by PD-L1 expression. Results: Both cohorts were well balanced in most of the features. Thirty-three percent of the tumors in the experimental group were PD-L1 positive, whereas 50% of them in the CG expressed PD-L1 (p = 0.06). Pathological CR was observed in 50% of the PD-L1 positive population, in contrast to a 2.8% in the PD-L1 negative in the NACT cohort (p < 0.01) as compared to the patients assigned to the DCV group (33.4% in PD-L1 positive vs 23.1% in PD-L1 negative population; p = 0.16). Among PD-L1 positive population, more pCR were seen in the CG than in the DCV group (50% vs 33.4%; p = 0.06). Within the PD-L1 negative population, more pCR were observed in the DCV group than in the CG (23.1% vs 2.8%; p < 0.05). With a median follow-up of 7 years, no significant differences were observed between the different subgroups neither in EFS (HR = 1.7; 0.42-6.8; p = 0.19) nor in OS (HR = 2.5; 0.56-11, p = 0.43). At 7 years, 20% and 14.4% of the patients relapsed according to the PD-L1 positive versus negative status respectively, and 10.78% versus 13.33% were dead. Conclusions: The benefit of DCV seems to be outstanding in the PD-L1 negative tumors that have a basal immune appropriate milieu. PD-L1 expression implies a more suppressed niche in which DCV are not able to stimulate antigen presentation and cell cytotoxic activity. PD-L1 positive population reach higher responses with both NACT±DCV than PD-L1 negative group, although the benefit seem to be higher in the NACT alone cohort. Further studies combining DVC+IPI together with NACT are needed. Clinical trial information: NCT01431196.

INTRODUCTION. Breast cancer (BC) is one of the most frequent cancers in women worldwide. Lack of therapeutic efficacy in some BC subtypes makes it necessary to develop new strategies. It is known that immune system plays a key role in the tumour control. Moreover, BC is immunogenic and the intensity of the immune response influences the clinical efficacy, and is correlated with a good prognosis. For that, manipulating the immune system to make it more effective could be an option. We design a clinical trial (NCT01431196) to evaluate the clinical efficacy of the combination of standard treatment with monocyte-derived dendritic cells vaccines preloaded with autologous tumour lysate (DCV) in stage I-III BC patients without HER2 overexpression. An in-depth immunological study was carried out. OBJECTIVES. To evaluate the immune response induced by the combination of neoadjuvant chemotherapy ± DCV ± radiation ± endocrine therapy in BC patients. METHODS. Serum and peripheral blood mononuclear cells (PBMCs) were collected from 20 luminal and triple negative BC patients before and after treatment. In serum, the presence of anti-tumour antibodies was studied by flow cytometry. PBMCs phenotype was evaluated by flow cytometry. In addition, functional studies were performed by incubating PBMCs with dendritic cells preloaded with autologous tumour lysate to evaluate specific immune response; specifically, T cell proliferation assay by [3H] thymidine incorporation and IFN-γ-producing cell assays by ELISPOT were done. Finally, the TCR clonality was assessed by flow cytometry. RESULTS. We identified IgG and IgM antibodies specific for breast cancer cell lines in the serum of 20 % and 40% of patients. Myeloid-derived suppressor cells decreased and NK cells increased with the treatment. Moreover, activation markers, such as HLADR, significantly increased in CD4+ and CD8+ lymphocytes after treatment, whereas PD1 and TIM3 decreased. In addition, an increase in the proliferation of specific T cells and in the number of IFN-γ producing cells after stimulation with tumor lysate pulsed DC was detected after treatment. Finally, very good clinical responders (pT4/5 with Miller&Payne classification and pN0) had higher TCR diversity index (DI) in CD4+ and CD8+ T cells in pretreatment samples: in CD8+ T cells, diversity index decreased in both groups after treatment, with higher difference in the VGCR group. CONCLUSION. Our study provides strong support that combined treatment induce humoral and cellular immune responses. We observed an activation of the immune system and a decrease in some immune checkpoints and MDSC. Finally, patients with a higher TCR-DI initially have a better clinical response and treatment induced oligoclonal activation of T cells. Citation Format: Laura Hato, Susana Inogés, Belén Pérez, Luis Mejías, Rodrigo Sánchez-Bayona, Marta Santisteban, Ascensión López. Immune response monitoring in breast cancer patients treated with neoadjuvant chemotherapy combined with dendritic cell vaccines [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-14-11.