Background: The use of patritumab (Daiichi Sankyo), an internalizing anti-HER3 monoclonal antibody, in combination with cetuximab and cisplatin was investigated in a cohort of patients with R/M HNSCC in a Phase II trial (NCT02633800). We investigated the EGFR-HER3 dimer (an established marker of cetuximab resistance within a previous study) in the circulating exosomes from patients enrolled in this trial. The objective was to identify non-invasive treatment stratification and longitudinal monitoring markers. Since antibody-directed cytotoxicity is involved as a mechanism of action for these therapies, exosomal microRNAs (miR21 and miR142), which promote the expansion of functional myeloid-derived suppressor cells (MDSC), as well as other suppressive adaptive and innate immune cell components are postulated as potential immune monitoring parameters. Methods: We extracted exosomes from patient serum at 2 timepoints: pretreatment (c1) and pre-cycle 2 (c2, day 1). Exosomes were stained with fluorescently labelled antibodies for fluorescence lifetime imaging microscopy (FLIM) to assess EGFR-HER3 dimerization by FRET. Serum exosomal miRs were analyzed using ddPCR. PBMCs were analyzed by flow cytometry with antibodies against CD3, CD4, CD8, CD25, CD127, CD45RO, CCR6, CCR7, and HLA-DR (T cells panel), or CD3, CD11b, CD14, CD16, CD19, CD24, CD27, CD33, CD38, and IgD (B cells/MDSC panel). We used Bayesian multivariate survival analysis, blinded, with overfitting prevention, to prospectively determine parameter contributions to risk. Results: Multivariate risk scores/signatures were derived that predicted either: progression free survival (PFS), with parameters including c1 MDSC, and c2 CD27-IgD-double-negative exhausted memory B cells; or RECIST response, with parameters including suppressive transitional B cells (c1 - c2 difference predicting response), and exosomal EGFR-HER3 dimer (c1 - c2 difference negatively predicting response). Conclusion: This study prospectively establishes an effect of exosomal and immunological factors on the efficacy of anti-HER/chemo combination therapies, paving the way for future stratification strategies that combine with immunological therapies. Clinical trial information: NCT02633800