Heterogeneity of Memory Marginal Zone B Cells

Abstract

The marginal zone (MZ) is largely composed of a unique subpopulation of B cells, the so-called MZ-B cells. At a molecular level, memory B cells are characterized by the presence of somatically mutated IGV genes. The earliest studies in the rat have documented the presence of hapten-specific MZ-B cells after immunization in the MZ. This work later received experimental support demonstrating that the IGHV-Cµ transcripts expressed by phenotypically defined splenic MZ-B cells (defined as CD90^negIgM^highIgD^low B cells) can carry somatic hypermutation. However, only a minor fraction (< 10%-20%) of these MZ-B cells is mutated and is considered to represent memory B cells. Memory B cells can either be class-switched (IgG, IgA, IgE), or non-class-switched (IgM) B cells. B cells in the MZ are a heterogeneous population of cells and both naïve MZ-B cells; class switched and unswitched memory MZ-B cells are present at this unique site in the spleen. Naïve MZ-B cells carry unmutated Ig genes, produce low-affinity IgM molecules and constitute a first line of defense against invading pathogens. Memory MZ-B cells express high-affinity Ig molecules, directed to (microbial) antigens that have been encountered. In this review, we report on the memory compartment of splenic MZ-B cells in the rat to provide insights into the origin and function of these memory MZ-B cells.

Keywords: immunoglobulin heavy (IGH) chain genes; marginal zone (MZ); memory B cells; rat; spleen.

FIG. 1

Histological structural organization of the rat spleen. The spleen is divided into red pulp (RP) and white pulp (WP) regions. The WP are further divided into B and T lymphocyte regions including the follicles, marginal zone and periarteriole lymphatic sheath (PALS). Visible in the WP are a secondary follicle i.e., a follicle containing a germinal center. The marginal zone (MZ) forms an interface between the RP and the WP. The MZ is further separated from the follicle and PALS by the marginal sinus.

FIG. 2

Structural organization of IGH and IGL in the rat. The above diagram demonstrates the organization of IGH gene segments position on chromosome 6q32-33 on the IGH locus of the Brown Norway (BN) rat. The IGH locus consists of an IGH variable region and heavy constant (CH) region. IGH variable region is grouped into three germline elements: IGHV, IGHD, and IGHJ gene segments respectively. Also depicted in the above figure are the recognition signal sequenced (RSS) flanking each of IGHV, IGHJ on one side, and IGHD on both sides. RSS consist of conserved heptamer and nonamer sequences, separated from each other by either a 12 or a 23 base-pair (bp) RSS spacer. Recombination of germline IGHV, IGHD, IGHJ (VDJ rearrangement) gene segments follow a 12/23 rule, i.e., VDJ rearrangement occurs only between a 12 bp RSS spacer and a 23 bp RSS spacer. Recombination activating Genes (RAG1/2) bind to RSS’s initiating VDJ rearrangement that results in the sequential random joining of a single gene segment of IGHD to IGHJ and IGHV to IGHDJ forming a rearranged IGHVDJ gene that encodes for the variable part of the immunoglobulin molecule. The IGHV gene is further arranged into CDR1 and CDR2 that are separated from each other by conserved framework regions (FR1, FR2, FR3). FR4 is completely encoded by the IGHJ gene. The CDR3 (junctional) region is the product of VDJ rearrangement. CH chain genes are separately joined by splicing to the rearranged IGHVDJ gene. The total number of gene segments, heptamer and nonamer sequences are designated by “n.”