A VpreB3 homologue in a marsupial, the gray short-tailed opossum, Monodelphis domestica

Evolution of surrogate light chain in tetrapods and the relationship between lengths of CDR H3 and VpreB tails

In the mammalian immune system, the surrogate light chain (SLC) shapes the antibody repertoire during B cell development by serving as a checkpoint for production of functional heavy chains (HC). Structural studies indicate that tail regions of VpreB contact and cover the third complementarity-determining region of the HC (CDR H3). However, some species, particularly bovines, have CDR H3 regions that may not be compatible with this HC-SLC interaction model. With immense structural and genetic diversity in antibody repertoires across species, we evaluated the genetic origins and sequence features of surrogate light chain components. We examined tetrapod genomes for evidence of conserved gene synteny to determine the evolutionary origin of VpreB1, VpreB2, and IGLL1, as well as VpreB3 and pre-T cell receptor alpha (PTCRA) genes. We found the genes for the SLC components (VpreB1, VpreB2, and IGLL1) only in eutherian mammals. However, genes for PTCRA occurred in all amniote groups and genes for VpreB3 occurred in all tetrapod groups, and these genes were highly conserved. Additionally, we found evidence of a new VpreB gene in non-mammalian tetrapods that is similar to the VpreB2 gene of eutherian mammals, suggesting VpreB2 may have appeared earlier in tetrapod evolution and may be a precursor to traditional VpreB2 genes in higher vertebrates. Among eutherian mammals, sequence conservation between VpreB1 and VpreB2 was low for all groups except rabbits and rodents, where VpreB2 was nearly identical to VpreB1 and did not share conserved synteny with VpreB2 of other species. VpreB2 of rabbits and rodents likely represents a duplicated variant of VpreB1 and is distinct from the VpreB2 of other mammals. Thus, rabbits and rodents have two variants of VpreB1 (VpreB1-1 and VpreB1-2) but no VpreB2. Sequence analysis of VpreB tail regions indicated differences in sequence content, charge, and length; where repertoire data was available, we observed a significant relationship between VpreB2 tail length and maximum DH length. We posit that SLC components co-evolved with immunoglobulin HC to accommodate the repertoire - particularly CDR H3 length and structure, and perhaps highly unusual HC (like ultralong HC of cattle) may bypass this developmental checkpoint altogether.

Comparative Aspects of Immunoglobulin Gene Rearrangement Arrays in Different Species

Studies in humans and mice indicate the critical role of the surrogate light chain in the selection of the productive immunoglobulin repertoire during B cell development. However, subsequent studies using mutant mice have also demonstrated that alternative pathways are allowed. Our recent investigation has shown that some species, such as pig, physiologically use preferential rearrangement of authentic light chains, and become independent of surrogate light chains. Here we summarize the findings from swine and compare them with results in other species. In both groups, allelic and isotypic exclusions remain intact, so the different processes do not alter the paradigm of B-cell monospecificity. Both groups also retained some other essential processes, such as segregated and sequential rearrangement of heavy and light chain loci, preferential rearrangement of light chain kappa before lambda, and functional κ-deleting element recombination. On the other hand, the respective order of heavy and light chains rearrangement may vary, and rearrangement of the light chain kappa and lambda on different chromosomes may occur independently. Studies have also confirmed that the surrogate light chain is not required for the selection of the productive repertoire of heavy chains and can be substituted by authentic light chains. These findings are important for understanding evolutional approaches, redundancy and efficiency of B-cell generation, dependencies on other regulatory factors, and strategies for constructing therapeutic antibodies in unrelated species. The results may also be important for explaining interspecies differences in the proportional use of light chains and for the understanding of divergences in rearrangement processes. Therefore, the division into two groups may not be definitive and there may be more groups of intermediate species.

Consequences of the different order of immunoglobulin gene rearrangements in swine

Swine use a reverse order of immunoglobulin chain rearrangement compared to humans and mice, and this altered and modified order should have measurable consequences. Here we perform new and defining experiments with developing and mature B cells, characterizing the B cell populations that do not exist in other species. First, we have finally confirmed that light chains κ and λ are rearranged and expressed on the surface before any heavy chain rearrangements using western-blot. And second, we have analyzed a pool of mature B cells on the single-cell level to demonstrate that many κ⁺ mature B cells carry λ transcripts. According to these findings, we believe that there may be more groups of mammals; one of which uses a pre-BCR-driven developmental pathway for B cell generation (like mice and humans), the second group uses a pre-BCR-independent one (like swine), and some may be even intermediate.

Immunoglobulin light chain κ precedes λ rearrangement in swine but a majority of λ+ B cells are generated earlier

Developmental pathways for B cell lymphogenesis are sufficiently known only in mice and humans. However, both of these species rearrange immunoglobulin heavy chains (IgH) before light chains (IgL) while IgL precedes IgH rearrangement in swine. We demonstrate here that this reversed order of rearrangements have some concealed consequences: (1) we confirmed that although IgLκ rearrangement is initial, most IgLλ⁺ B cells are generated earlier and before IgH rearrangements, while most IgLκ⁺ B cells later and after IgH rearrangements, (2) the second IgLκ rearrangement can occur after IgLλ rearrangement, (3) early formed B cells bear only single in-frame IgH rearrangements, (4) many IgLκ⁺ B cells carry IgLλ rearrangements that can be productive and occurring on both alleles in one cell, and (5) although VpreB and λ5 genes are present in swine, they are preferentially expressed in non-B cells. In summary, our findings reveal that swine use an alternative B cell developmental pathway as compared to mice and humans.

Immunogenetics of marsupial B-cells

Marsupials and eutherians are mammals that differ in their physiological traits, predominately their reproductive and developmental strategies; eutherians give birth to well-developed young, while marsupials are born highly altricial after a much shorter gestation. These developmental traits also result in differences in the development of the immune system of eutherian and marsupial species. In eutherians, B-cells are the key to humoral immunity as they are found in multiple lymphoid organs and have the unique ability to mediate the production of antigen-specific antibodies in the presence of extracellular pathogens. The development of B-cells in marsupials has been reported and hypothesised to be similar to that of eutherians, except that haematopoiesis occurs in the liver, postpartum, until the bone marrow fully matures. In eutherians, specific genes are linked to specific stages in B-cell development, maturation, and differentiation processes, and have been identified including immunoglobulins (heavy and light chains), cluster of differentiation markers (CD10, 19, 34 and CD79α/β), signal transduction molecules (BTK, Lyn and Syk) and transcriptional regulators (EBF1, E2A, and Pax5). This review aims to discuss the known similarities and differences between marsupial and eutherian B-cells, in regards to their genetic presence, homology, and developmental stages, as well as to highlight the areas requiring further investigation. By enhancing our understanding of the genes that are involved with B-cells in the marsupial lineage, it will, in turn, aid our understanding of the marsupial immune system and support the development of specific immunological reagents for research and wildlife conservation purposes.

Haematopoiesis in Marsupials

Marsupials are a group of mammals that give birth to immature young lacking mature immune tissues at birth, and are unable to mount their own specific immune defence. Their immune tissues develop in a non-sterile ex-utero environment unlike that of eutherian mammals such as ourselves. Marsupials are therefore ideal models for studying the development of immune tissues, in particular haematopoiesis, yet relatively little has been investigated. Most studies have been restricted to histological or immunohistological studies, however some factors likely to be involved, based on eutherian studies in haematopoiesis, have been isolated and characterised, including a few key markers, and some cell signaling and regulation molecules, mostly involved in lymphocytopoiesis. However the role of many molecules in haematopoiesis is largely presumed. We currently lack much of the rudimentary information regarding time of appearance and expression levels of these molecules, and no functional studies have been conducted. This paper reviews our knowledge of marsupial haematopoiesis to date, and highlights the need for future research in marsupials to gain further insights into the evolution of haematopoiesis.

Pre-B Lymphocyte Protein 3 (VPREB3) Expression in the Adrenal Cortex: Precedent for non-Immunological Roles in Normal and Neoplastic Human Tissues

The pre-B lymphocyte protein 3 (VPREB3) is expressed during B cell differentiation and in subsets of mature B lymphocytes and is mainly found in bone marrow and lymphoid tissue germinative centers. So far, its function in B cells remains to be clarified. The messenger RNA (mRNA) of VPREB3 was previously detected in aldosterone-producing adenomas (APA); however, further information about this protein in human adrenocortical cells and tissues is currently unavailable. Therefore, in the present study, we, for the first time, investigate the protein expression of VPREB3 in human adrenocortical tissues. In addition, we approach the previously suggested similarities in expression patterns of aldosterone-producing cells and Purkinje neurons. Immunohistochemical analysis of VPREB3 was performed in 13 nonpathological adrenals (NA), 6 adrenal glands with idiopathic hyperaldosteronism (IHA), 18 APA, 5 cortisol-producing adenomas (CPA), and 5 nonpathological human cerebellum specimens. The mRNA levels of VPREB3, steroidogenic enzymes, and other aldosterone biosynthesis markers were detected in 53 APA samples using real-time RT-PCR (qPCR) and compared to the clinical data of APA patients. In our results, the VPREB3 protein was diffusely detected in APA, partially or weakly detected in CPA, and immunolocalized in the zona glomerulosa of NA and IHA, as well as in the cytoplasm of cerebellar Purkinje cells. In APA, VPREB3 mRNA levels were significantly correlated to plasma aldosterone (P = 0.026; R = 0.30), KCNJ5 mutations (P = 0.0061; mutated 34:19 wild type), CYP11B2 (P < 0.0001; R = 0.65), Purkinje cell protein 4 (PCP4; P < 0.0001; R = 0.53), and voltage-dependent calcium channels CaV1.3 (P = 0.023; R = 0.31) and CaV3.2 (P = 0.0019; R = 0.42). Based on our data, we hypothesize a possible role for VPREB3 in aldosterone biosynthesis, and present ideas for future functional studies.

Isoforms of the CD79 signal transduction component of the macropod B-cell receptor

B cell responses and their concomitant signal transduction pathways are not well understood in marsupial mammals, despite the availability of gene expression data for key immunoglobulin genes and for elements of the CD79a/CD79b heterodimer signalling complex for two model marsupials. Broader studies of factors that influence B cell responses are still hampered by a lack of species-specific reagents and there are few reports of other factors that influence gene expression such as the potential for splice variants in BCR components, which may influence immune signalling pathways. In this study, we characterise CD79a and CD79b genes in the endangered macropod marsupial, Onychogalea fraenata (the bridled nailtail wallaby) and show that domains and residues important for the structural and functional integrity of both monomers are conserved in this species, consistent with results previously reported for the closely-related macropod, Macropus eugenii (the tammar wallaby). We extend this work to report the detection of splice variants for CD79a and CD79b in wallaby species; three CD79a isoforms and one CD79b isoform. Of these, two CD79a isoforms and the CD79b isoform have not been reported in any other mammalian species.

Immunoglobulin light chains in medaka (Oryzias latipes)

The gene segments encoding antibodies have been studied in many capacities and represent some of the best-characterized gene families in traditional animal disease models (mice and humans). To date, multiple immunoglobulin light chain (IgL) isotypes have been found in vertebrates and it is unclear as to which isotypes might be more primordial in nature. Sequence data emerging from an array of fish genome projects is a valuable resource for discerning complex multigene assemblages in this critical branch point of vertebrate phylogeny. Herein, we have analyzed the genomic organization of medaka (Oryzias latipes) IgL gene segments based on recently released genome data. The medaka IgL locus located on chromosome 11 contains at least three clusters of IgL gene segments comprised of multiple gene assemblages of the kappa light chain isotype. These data suggest that medaka IgL gene segments may undergo both intra- and inter-cluster rearrangements as a means to generate additional diversity. Alignments of expressed sequence tags to concordant gene segments which revealed each of the three IgL clusters are expressed. Collectively, these data provide a genomic framework for IgL genes in medaka and indicate that Ig diversity in this species is achieved from at least three distinct chromosomal regions.

Marsupial immunology bounding ahead

Marsupial immune responses were previously touted as ‘primitive’ but we now know that the marsupial immune system is complex and on par with that of eutherian mammals. In this manuscript we review the field of marsupial immunology, focusing on basic anatomy, developmental immunology, immunogenetics and evolution. We concentrate on advances to our understanding of marsupial immune gene architecture, made possible by the recent sequencing of the opossum, tammar wallaby and Tasmanian devil genomes. Characterisation of immune gene sequences now paves the way for the development of immunological assays that will allow us to more accurately study health and disease in marsupials.