Diversity of Rainbow Trout Blood B Cells Revealed by Single Cell RNA Sequencing

Pedro Perdiguero,Carolina Tafalla,Esther Morel

doi:10.3390/biology10060511

Abstract

Simple SummaryAlthough evolutionarily jawed fish constitute the first group of animals in which a complete adaptive immune system based on immunoglobulins (Igs) is present, many structural immune differences between fish and mammals predict important functional and phenotypical differences between B cells in these two animal groups. However, to date, very few tools are available to study B cell heterogeneity and functionality in fish. Hence, thus far, antibodies targeting the different Igs have been almost exclusively applied as tools to investigate B cell functionality in fish. In the current study, we used the newly developed 10× Genomics single cell RNA sequencing technology and used it to analyze the transcriptional pattern of single B cells from peripheral blood. The results obtained provide us with a transcriptional profile at single cell level of what seem to correspond to different B cell subsets or B cells in different stages of maturation or differentiation. The information provided will not only help us understand the biology of teleost B cells, but also provides us with a repertoire of potential markers that could be used in the future to differentiate trout B cell subsets. Single-cell sequencing technologies capable of providing us with immune information from dozens to thousands of individual cells simultaneously have revolutionized the field of immunology these past years. However, to date, most of these novel technologies have not been broadly applied to non-model organisms such as teleost fish. In this study, we used the 10× Genomics single cell RNA sequencing technology and used it to analyze for the first time in teleost fish the transcriptional pattern of single B cells from peripheral blood. The analysis of the data obtained in rainbow trout revealed ten distinct cell clusters that seem to be associated with different subsets and/or maturation/differentiation stages of circulating B cells. The potential characteristics and functions of these different B cell subpopulations are discussed on the basis of their transcriptomic profile. The results obtained provide us with valuable information to understand the biology of teleost B cells and offer us a repertoire of potential markers that could be used in the future to differentiate trout B cell subsets.

Highlights

To date, most studies have agreed on the fact that adaptive immunity appeared during the early stages of vertebrate evolution, most likely in the disappeared placoderms [1]
To acquire a transcriptomic profile of rainbow trout peripheral blood B cells at singlecell resolution, leucocyte isolation following sorting of lymphoid MHC II+ cells was carried out using blood obtained from three independent unstimulated fish
Our analysis focused in B cells seems to indicate that CXCR4 relates to an advanced state of maturation, but with different rainbow trout CXCR4 molecules carrying out a differential regulation of specific B cell subpopulations

Summary

Introduction

Most studies have agreed on the fact that adaptive immunity appeared during the early stages of vertebrate evolution, most likely in the disappeared placoderms [1]. GCs, formed in mammals during the immune response, promote the close collaboration between proliferating antigen-specific B cells, T follicular helper cells, and specialized follicular dendritic cells (DCs) In this environment, B cells divide in response to antigens and acquire the capacity to differentiate into antibody-secreting cells (ASCs), reaching a terminal state of plasma cells or memory B cells, both having the capacity to secrete high affinity antibodies. B cells divide in response to antigens and acquire the capacity to differentiate into antibody-secreting cells (ASCs), reaching a terminal state of plasma cells or memory B cells, both having the capacity to secrete high affinity antibodies It is in these sites, that B cells undergo class switch recombination (CSR) and replace the heavy chain of IgM for IgG, IgA or IgE, antibodies with higher affinity and different effector functions. In the absence of GCs or specialized Igs, whether teleost B cells differentiate to plasma cells or memory B cells equivalent to those found in mammals is still a matter of debate

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Conclusion