Background and Aims:
Recent studies have reported that B cells and antibodies can affect post-myocardial infarction healing and favor the progression of ischemic heart failure, but the antigen specificities remain elusive. To produce high-affinity antibodies, B cells typically undergo serial rounds of somatic hypermutation and selection processes within the secondary lymphoid organs in the so-called germinal center reactions. Thus, we sought to i) assess whether experimental MI triggers germinal center (GC) B cells responses in the heart-draining lymph nodes and ii) to characterize the repertoire of B cell receptors (BCRs, membrane-bound immunoglobulins) expressed by the B cells activated after MI.

Methods:
Experimental MI was induced in WT C57BL/6J mice after permanent ligation of the left anterior descending artery (LAD). The GC reactions in the heart-draining lymph nodes and spleen were monitored by flow cytometry and by single-cell-RNA/BCR sequencing on days 5 and 14 post-operation.

Results:
Flow cytometry and transcriptomic findings revealed a significant increase in the number of GC B cells from day 5 to day 14 in the heart-draining lymph nodes after MI. Sham operated animals also showed some GC expansion on day 14, but not on day 5 post-thoracotomy. Gene set analyses performed on 3962 sequenced B cells enabled the identification of 10 different B cell clusters among which we could identify the subsets of our interest: light and dark zone GC B cells and plasma cells, marked by high expression of Aicda, Mki67, Fas, Sdc1, Xbp1, Irf4 and Prdm1 transcripts. Expanded plasma cells were observed in the heart-draining lymph nodes on day 14 post-MI, but not in sham-operated controls. Moreover, GC B cells showed increasing levels of clonal expansion over time, marked by increased numbers of proliferating B cells expressing the same antigen receptor, indicating the development of antigen-driven B cell responses in the context of MI. After performing analyses of immunoglobulin heavy and light chain rearrangements, we identified 9 distinct MI-related B cell lineages in the heart-draining lymph nodes and spleen expressing mutated BCRs, suggestive of an MI-triggered antibody affinity maturation process.

Conclusions:
Our findings revealed that experimental MI triggers clonal expansion of GC B cells and we identified a distinct set of BCRs undergoing somatic hypermutation, which is a fundamental step towards the production of high-affinity antibodies. These findings will pave the way for future mechanistic studies aimed at uncovering the antigen specificity and functional relevance of these MI-triggered antibodies.