Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• Allergic disease

• Memory B cells

Main Findings

Allergic disease is driven by IgE-producing plasma cells that can produce allergen-specific IgE antibodies that bind to high-affinity receptors on mast cells and basophils and cause their degranulation upon allergen crosslinking. Allergies can persist for many years or even lifetimes due to immunological memory, but the cell type that holds this allergen-specific memory is poorly characterized. As IgE-producing plasma cells are short-lived and IgE memory B cells (MBCs) are rare, Koenig, J. et al. sought to identify and characterize the primary reservoir of allergen-specific IgE plasma cells.

In more recent years, it was found that allergen-specific memory can be kept by non-IgE MBCs and that the isotype of these MBCs are mainly IgG1. However, the phenotype of these MBCs were not well described. Through utilizing novel and pre-existing single-cell datasets in several different human models of allergy, they identified and described a novel memory B cell subset that holds allergen-specific IgE memory. With this high-dimensional profiling, they found a subset of type 2 polarized MBCs they label as “MBC2s” that are CD23hi, IL-4Rαhi, CD32low at the transcriptional and surface protein levels. These cells also are distinguishably by their expression of IgE germline transcript (εGLT). They are polarized by IL-4, express antigen presentation machinery and adaptive immune activation gene signatures and have a transcriptional program that retains them in a memory fate. These MBC2 clusters were enriched in IgG1 and IgG4 isotype expression, providing evidence that MBC isotype is not conclusive alone to explain the ability of IgG1 MBCs to hold IgE memory.

When they investigated patients with allergic rhinitis and peanut allergy via Smart-Seq2 single-cell RNA sequencing and flow cytometry, they found that their allergen-specific MBCs were mainly MBC2s. They compared this to RBD-specific memory B cells from patients who have been vaccinated or infected with SARS-Cov-2, and found that very few of these viral-specific MBCs demonstrated this MBC2 phenotype. They also were able to identify murine MBC2s that were homologues of human MBC2s in murine models of allergy.

Finally, they provided compelling human in vivo evidence that MBC2s are primary clonal relatives of allergen-specific IgE. Through clonal tracking, they found that MBC2s were the precursors to allergen-specific IgE plasma cells after month of allergen sublingual immunotherapy in birch-allergic individuals.

Limitations

• While the authors identified mouse MBC2s that are likely homologues to human MBC2s based on surface protein and transcriptomic data, they did not further prove if they differentiate into to allergen specific IgE plasma cells that mediate disease. They made the conclusion that these murine MBC2s hold allergen-specific IgE memory based on their observation that there were higher proportions of class-switched CD23hi IL-4Rαhi Ova-specific MBCs in type 2 sensitization models vs. Type 1 sensitization models. However, they could go further by clonally tracking these allergen-specific MBC2s as they differentiate into plasma cells and assessing if these plasma cells produce IgE antibodies in response to allergic challenge. Further mouse experiments could more conclusively definitely demonstrate these MBC2s as being the primary reservoir of allergen-specific IgE plasma cells.

• Further experiments could be conducted in mice to better understand cytokine signals, milieus, and/or pathways that drive IgE plasma cell production from MBC2s. While they identified Il-4 as being important to produce MBC2s, they did not delve into what signals are required for these MBC2s to class switch to IgE plasma cells.

• While they observed that both allergic and non-allergic individuals have similar frequencies of IGHE+ MBC2 in their peripheral blood, they do not elaborate on what the role of non-allergen specific IGHE+ MBC2s may be in non-allergic people. This could be a question that could be further probed in their type 1 and 2 Ova sensitization mouse models by understanding what cell types the non-Ova-specific MBC2s differentiate into in these two models.

• Furthermore, it could benefit the authors to attempt various methods of depleting (e.g. bispecific antibody) or hindering the production of these allergen-specific MBC2s in their mouse models of allergy to further solidify and demonstrate their importance in allergic pathology.

Significance/Novelty

Their study’s findings open the possibility for novel treatment options for allergy. Current existing allergy treatments are limited to allergen avoidance, medications to alleviate symptoms, emergency epinephrine, or allergy immunotherapy (allergy shots or sublingual allergen exposure). Their characterization of a novel memory B cell subset that holds allergen-specific IgE memory excitingly provides a new potential therapeutic target for IgE-mediated allergic disease. Additionally, Koenig, J. et al.  begin to elucidate potential pathways that may drive production of MBC2s. They identify IL-4 as driving expression of CD23, IL-4Ralpha, and the IGHE locus. Better understanding this biology and their development can help us to potentially reprogram allergen-specific memory.

Beyond allergy, these MBC2s may demonstrate importance for other contexts involving type 2 immune responses and pathologies involving IgE. They may be important for immunity against helminths or play a role in autoimmunity or atherosclerosis.

Credit

Reviewed by Michelle Tran (Mount Sinai) as part of a cross-institutional journal club between the Icahn School of Medicine at Mount Sinai, the University of Oxford, the Karolinska Institute and the University of Toronto.