Recent studies show that not all the COPD patients, but only those having emphysema present a prominent lung B cell molecular signature irrespectively of the degree of airflow limitation. Our preliminary data show that lung and circulating B cell responses are upregulated in COPD patients with greater vs. lesser emphysema within the same COPD severity stage. We will test the central hypothesis that emphysema is characterized by changes in B cell transcriptome which underlie exaggerated and off-targeted B cell responses contributing to lung destruction. In Aim 1, we will test the hypothesis that B cell, lymphoid follicles (LFs), and B cell product counts, and specific LF B cell clonal and post-transcriptional changes are increased in the emphysematous lung vs. lung with lesser emphysema. In Aim 1A, we will immunostain, using both conventional immunostaining and image mass cytometry, COPD and control lungs and correlate the numbers of B cells, LFs, and other cells involved in B cells responses, with the extent of computed tomography (CT)-defined lobar emphysema. In Aim 1B, we will use laser capture microdissection to extract LF B cells from COPD and control lung sections to then: 1) assess the clonality of the LFs; and 2) study the rearrangement of the B cell receptor (BCR) using a next-generation sequencing (NGS) assay. In Aim 2, we will test the hypothesis that an exaggerated B cell activation, associated with transcriptomic changes, is present in the emphysematous lung. We will isolate lung B cells from COPD patients and controls in order to: 1) test B cell activation and immunoglobulin (Ig) production in response to antigens using in vitro cultures and ELISPOT assays; 2) characterize the B cell pool using flow cytometry (Aim 2A); 3) study lung B cell transcriptomes and BCR rearrangements using single-cell RNA sequencing (scRNAseq); 4) generate monoclonal Igs and identify their binding targets in the lung with mass spectrometry (Aim 2B). We will correlate the readouts of B cell activation in the lung with the extent of CT scan-defined lobar emphysema. In Aim 3, we will test the hypothesis that circulating B cell function and phenotypes reflect lung B cell features in correlation studies relating to the extent of emphysema. In Aim 3A, we will: 1) test blood B cell activation and Ig production in response to antigens using in vitro cultures and ELISPOT assays; 2) characterize the blood B cell pool using flow cytometry; and 3) study blood B cell transcriptomes and BCR rearrangements using scRNAseq. In Aim 3B, we will use COPD and control plasma to: 1) quantify the blood Ig isotypes and identify the blood Ig binding targets in the lung using mass spectrometry; and 2) measure the levels of B cell- related biomarkers and IgG reactivity against lung matrix. We will correlate the readouts of B cell activation in blood and lung from the same subject, with the extent of CT scan-defined emphysema. These studies will define the nature of the B cell responses in a clinical phenotype (likely emphysema) which could benefit from therapies targeting B cells or B cell products, leading to earlier and more personalized therapeutic intervention for COPD.