B-1 and B-2 B-cells in the pig cannot be differentiated by expression of CD5

Single-cell transcriptomic analysis reveals transcriptional and cell subpopulation differences between human and pig immune cells

BACKGROUND

The pig is a promising donor candidate for xenotransplantation. Understanding the differences between human and swine immune systems is critical for addressing xenotransplant rejection and hematopoietic reconstitution. The gene transcriptional profile differences between human and pig immune cell subpopulations have not been studied. To assess the similarities and differences between pigs and humans at the levels of gene transcriptional profiles or cell subpopulations are important for better understanding the cross-species similarity of humans and pigs, and it would help establish the fundamental principles necessary to genetically engineer donor pigs and improve xenotransplantation.

OBJECTIVE

To assess the gene transcriptional similarities and differences between pigs and humans.

METHODS

Two pigs and two healthy humans' PBMCs were sorted for 10 × genomics single-cell sequence. We generated integrated human-pig scRNA-seq data from human and pig PBMCs and defined the overall gene expression landscape of pig peripheral blood immune cell subpopulations by updating the set of human-porcine homologous genes. The subsets of immune cells were detected by flow cytometry.

RESULTS

There were significantly less T cells, NK cells and monocytes but more B cells in pig peripheral blood than those in human peripheral blood. High oxidative phosphorylation, HIF-1, glycolysis, and lysosome-related gene expressions in pig CD14+ monocytes were observed, whereas pig CD14+ monocytes exhibited lower levels of cytokine receptors and JAK-STAT-related genes. Pig activated CD4+T cells decreased cell adhesion and inflammation, while enriched for migration and activation processes. Porcine GNLY+CD8+T cells reduced cytotoxicity and increased proliferation compared with human GNLY+CD8+T cells. Pig CD2+CD8+γδT cells were functionally homologous to human CD2+CD4+ γδT cells. Pig CD2-CD8-γδT cells expressed genes with quiescent and precursor characteristics, while CD2-CD8+γδT cells expressed migration and memory-related molecules. Pig CD24+ and CD5+B cells are associated with inflammatory responses.

CONCLUSION

Our research with integrated scRNA-seq assays identified the different distribution of pig immune cell subpopulations and the different transcriptional profiles of human and pig immune cells. This study enables a deeper understanding of the development and function of porcine immune cells.

Human VH4-34 antibodies derived from B1 cells are more frequently autoreactive than VH4-34 antibodies derived from memory cells

Human B1 cells produce natural antibodies characterized by overutilization of heavy chain variable region VH4-34 in comparison to other B cell populations. VH4-34-containing antibodies have been reported to be autoreactive and to be associated with lupus and other autoimmune dyscrasias. However, it has been unclear to what extent VH4-34 antibodies manifest autoreactivity in B1 cells or other B cell populations-in other words, are VH4-34 containing antibodies autoreactive wherever found, or mainly within the B1 cell population? To address this issue we sort purified single human B1 and memory B cells and then amplified, sequenced, cloned and expressed VH4-34-containing antibodies from 76 individual B cells. Each of these antibodies was tested for autoreactivity by HEp-2 IFA and autoantigen ELISA. Antibodies were scored as autoreactive if positive by either assay. We found VH4-34 antibodies rescued from B1 cells were much more frequently autoreactive (14/48) than VH4-34 antibodies rescued from memory B cells (2/28). Among B1 cell antibodies, 4 were HEp-2+, 6 were dsDNA+ and 4 were positive for both. Considering only HEp-2+ antibodies, again these were found more frequently among B1 cell VH4-34 antibodies (8/48) than memory B cell VH4-34 antibodies (1/28). We found autoreactivity was associated with greater CDR3 length, as expected; however, we found no association between autoreactivity and a previously described FR1 "hydrophobic patch". Our results indicate that autoreactive VH4-34-containing antibodies tend to reside within the human B1 cell population.

Effects of freezing storage on the stability of maternal cellular and humoral immune components in porcine colostrum

The placental structure of sows is epitheliochorial and prevents maternal serum immunoglobulin transfer to the fetus; therefore, the piglet relies on the ingestion of colostrum to acquire passive immunity. Colostral antibody-mediated and cell-mediated immunity contribute to immunity in piglets. However, little is known about the effects of freezing at -20 °C on colostral immune components during short-term storage, whether this will somehow compromise the acquisition of passive immunity of newborn piglets fed with this colostrum and the humoral immunity in porcine colostrum, and to possible shifts in immunological levels in colostrum collections during the colostral period. Based on the average concentration of immunoglobulin, frozen and fresh colostrum did not differ significantly. Overall, there were no storage differences in total macrophages, granulocytes, and NK cells. However, the frozen colostrum presented T lymphocyte subsets and B lymphocytes significantly lower than the fresh colostrum (p ≤ 0.05). Therefore, to sustain higher piglet survival rates, B cells may be a selective strategy to ensure immune defense to neonatal piglets. According to our findings, colostrum can be stored by freezing at -20 °C for up to 30 days and surplus porcine colostrum can be collected from the sow up to eight hours after the start of farrowing.

Identification of IL-10 competent B cells in swine

Progress in the phenotypic characterisation of porcine B cells is ongoing, with recent advances in the identification of B1 cell subsets and plasma cells. However, regulatory B cells, commonly identified by interleukin (IL)-10 production, have not been studied in pigs so far. Here we investigate IL-10 expression in B cell subsets in response to CpG-oligodeoxynucleotides, phorbol 12-myristate 13-acetate and ionomycin stimulation in vitro. Our results reflect similar findings in human and mice. We identify a small subset of IL-10 competent B cells, present within both porcine B1 and B2 cell subsets across blood, spleen, mediastinal lymph nodes and lung tissue, with varied differentiation statuses. The capacity for IL-10 production coincided with CD95 expression, suggesting an activated phenotype of IL-10 competent B cells. These findings support the emerging paradigm that B cell IL-10 production is a function of various B cell subsets influenced by activation history and microenvironmental factors.

Distribution difference of colostrum-derived B and T cells subsets in gilts and sows

Piglets are highly vulnerable to infections, but colostrum provides them with some protection. The function of colostrum components is unknown, as is if the amount and subsets of leukocytes in colostrum differ between gilts and sows. This study serially characterized leukocyte populations in colostrum for differential leukocyte counts. Differences in humoral and cellular composition of colostrum between 40 gilts and 40 sows (parities orders 3-4) from a commercial herd were examined. Flow cytometry is a useful tool to identify and quantify leukocyte subsets in sow colostrum. Overall, there were no (p ≥ 0.05) parity differences in total macrophages, granulocytes, and T and B cells. However, the sows' colostrum presented significantly higher (p ≤ 0.05) T lymphocyte subsets than gilts, such as central memory CD4+T cells, effector memory CD4+T cells, and central memory CD8+T cells. Among B-lymphocytes, percentages of SWC7+CD5+ cells were significantly higher in sow colostrum than in that of gilts. As expected, IgG concentrations were significantly higher in sows than in gilts. Colostrum from sows had significantly greater mitogenic activity than colostrum from gilts and this fact can be associated with the potential to accelerate the maturation of a newborn's gastrointestinal tract. Our findings suggest that parity order may be one among other factors influencing the cell population and, consequently, the immune adaptive response in piglets that induces neutralizing antibodies and cellular immune responses to antigens.

Porcine B Cell Subset Responses to Toll-like Receptor Ligands

Toll-like receptors (TLR) triggering of B cells are known to promote B cell expansion, differentiation of B cells into antibody-producing and memory cells, but the TLR responses of porcine B cells is poorly characterized. Therefore, this study investigated the response pattern of porcine B cell subsets to a large collection of TLR ligands and demonstrates that the TLR2 ligand Pam3Cys-SK4 and the TLR7/8 ligands gardiquimod and resiquimod are particularly efficient at inducing proliferation, CD25 and CCR7. This activation was also determined in B-cell subpopulations including a CD21⁺IgM⁺ subset, an IgG⁺ subset and two putative B1-like subsets, defined as CD21^-IgM^highCD11R1⁺CD11c⁺CD14⁺ and CD21^-IgM^high CD11R1^-CD11c⁺CD14^- B cells. The latter two were larger and expressed higher levels of CD80/86 and spontaneous phospholipase C-γ2 phosphorylation. All porcine B-cell subsets were activated by TLR2, TLR7, and TLR9 ligands. Naïve and memory conventional B cells responded similar to TLR ligands. The CD11R1⁺ B1-like subset had the highest proliferative responses. While both B1-like subsets did not spontaneously secrete IgM, they were the only subsets to produce high level of TLR-induced IgM. Similar to polyclonal IgM responses, memory B cells were efficiently induced to produce specific antibodies by CpG oligodinucleotide, resiquimod, and to a weaker extend by Pam3Cys-SK4. Depletion of plasmacytoid dendritic cells (pDCs) enhanced TLR-induced antibodies. The same set of TLR ligands also induced CD40 on cDCs, pDCs, and monocytes with the exception of TLR4 ligand being unable to activate pDCs. Gardiquimod and resiquimod were particularly efficient at inducing CCR7 on pDCs. Porcine B cells expressed high levels of TLR7, but relatively little other TLR mRNA. Nevertheless, TLR2 on B cells was rapidly upregulated following stimulation, explaining the strong responses following stimulation. Subset-specific analysis of TLR expression demonstrated a comparable expression of TLR2, TLR7, and TLR9 in all B cell subsets, but TLR3 was restricted to B1-like cells, whereas TLR4 was only expressed on conventional B cells, although both at low levels. Altogether, our data describe porcine innate B1-like cells, and how different B cell subsets are involved in innate sensing.

Developmental expression of B cell molecules in equine lymphoid tissues

Identification and classification of B cell subpopulations has been shown to be challenging and inconsistent among different species. Our study tested aspects of ontogeny, phenotype, tissue distribution, and function of equine CD5^hi B cells, which represented a greater proportion of B cells early in development and in the peritoneal cavity. CD5^hi and CD5^lo B cells differentially expressed B cell markers (CD2, CD21, IgM) measured using flow cytometry, but similar mRNA expression of signature genes (DGKA, FGL2, PAX5, IGHM, IL10) measured using quantitative RT-PCR. Sequencing lambda light chain segments revealed that CD5^hi B cells generated diverse immunoglobulin repertoires, and more frequently bound to fluorescence-labeled phosphorylcholine. This study shows developmental characteristics and tissue distribution of a newly described subpopulation of B cells in the horse.

Antibody repertoire development in fetal and neonatal piglets. XXIV. Hypothesis: The ileal Peyer patches (IPP) are the major source of primary, undiversified IgA antibodies in newborn piglets

The ileal Peyers patches (IPP) of newborn germfree (GF) piglets were isolated into blind loops and the piglets colonized with a defined probiotic microflora. After 5 weeks, IgA levels in the intestinal lavage (IL) of loop piglets remained at GF levels and IgM comprised ∼70% while in controls, IgA levels were elevated 5-fold and comprised ∼70% of total Igs. Loop piglets also had reduced serum IgA levels suggesting the source of serum IgA had been interrupted. The isotype profile for loop contents was intermediate between that in the IL of GF and probiotic controls. Surprisingly, colonization alone did not result in repertoire diversification in the IPP. Rather, colonization promoted pronounced proliferation of fully switched IgA(+)IgM(-) B cells in the IPP that supply early, non-diversified "natural" SIgA antibodies to the gut lumen and a primary IgA response in serum.

Development of gut immunoglobulin A production in piglet in response to innate and environmental factors

The current review focuses on pre- and post-natal development of intestinal immunoglobulin A (IgA) production in pig. IgA production is influenced by intrinsic genetic factors in the foetus as well as extrinsic environmental factors during the post-natal period. At birth, piglets are exposed to new antigens through maternal colostrums/milk as well as exogenous microbiota. This exposure to new antigens is critical for the proper development of the gut mucosal immune system and is characterized mainly by the establishment of IgA response. A second critical period for neonatal intestinal immune system development occurs at weaning time when the gut environment is exposed to new dietary antigens. Neonate needs to establish oral tolerance and in the absence of protective milk need to fight potential new pathogens. To improve knowledge about the immune response in the neonates, it is important to identify intrinsic and extrinsic factors which influence the intestinal immune system development and to elucidate their mechanism of action.

Human B-1 cells take the stage

B-1 cells play critical roles in defending against microbial invasion and in housekeeping removal of cellular debris. B-1 cells secrete natural antibody and manifest functions that influence T cell expansion and differentiation and in these and other ways differ from conventional B-2 cells. B-1 cells were originally studied in mice where they are easily distinguished from B-2 cells, but their identity in the human system remained poorly defined for many years. Recently, functional criteria for human B-1 cells were established on the basis of murine findings, and reverse engineering resulted in identification of the phenotypic profile, CD20(+)CD27(+)CD43(+)CD70(-), for B-1 cells found in both umbilical cord blood and adult peripheral blood. Human B-1 cells may contribute to multiple disease states through production of autoantibody and stimulation/modulation of T cell activity. Human B-1 cells could be a rich source of antibodies useful in treating diseases present in elderly populations where natural antibody protection may have eroded. Manipulation of human B-1 cell numbers and/or activity may be a new avenue for altering T cell function and treating immune dyscrasias.

The enigma of the lower gut-associated lymphoid tissue (GALT)

Artiodactyls possess GALT that appears in fetal life and is located at the extreme end of the ileum. These IPP contain mostly B cells and involute early in postnatal life. Rabbits have a similarly located lymphoid organ, called the sacculus rotundus. Studies in sheep and rabbits have led to the concept that the lower hindgut GALT represents primary lymphoid tissue for B cells and is necessary for normal B cell development, analogous to the bursa of Fabricius. This review traces the history of the observations and theories that have led to the existing concept concerning the role of lower GALT. We then review recent data from piglets with resected IPP that challenges the concept that the IPP is primary B cell lymphoid tissue and that artiodactyls and rabbits are members of the GALT group in the same context as gallinaceous birds. Eliminating the IPP as the primary lymphoid tissue for B cells leads to the hypothesis that the IPP acts as first-responder mucosal lymphoid tissue.

Human B1 cells in umbilical cord and adult peripheral blood express the novel phenotype CD20+ CD27+ CD43+ CD70-

Human B1 cells consist of CD20⁺CD27⁺CD43⁺CD70⁻ cells bearing a skewed B cell receptor repertoire, and are present in umbilical cord and adult peripheral blood.

B1 cells differ in many ways from conventional B cells, most prominently in the production of natural immunoglobulin, which is vitally important for protection against pathogens. B1 cells have also been implicated in the pathogenesis of autoimmune dyscrasias and malignant diseases. It has been impossible to accurately study B1 cells during health and illness because the nature of human B1 cells has not been successfully defined. This has produced controversy regarding the existence of human B1 cells. Here, we determined the phenotype of human B1 cells by testing sort-purified B cell fractions for three fundamental B1 cell functions based on mouse studies: spontaneous IgM secretion, efficient T cell stimulation, and tonic intracellular signaling. We found that a small population of CD20⁺CD27⁺CD43⁺ cells present in both umbilical cord and adult peripheral blood fulfilled these criteria and expressed a skewed B cell receptor repertoire. These B cells express little or no surface CD69 and CD70, both of which are markedly up-regulated after activation of CD20⁺CD27⁻CD43⁻ (naive) and CD20⁺CD27⁺CD43⁻ (memory) B cells. This work identifies human B1 cells as CD20⁺CD27⁺CD43⁺CD70⁻. We determined that the proportion of B1 cells declines with age, which may contribute to disease susceptibility. Identification of human B1 cells provides a foundation for future studies on the nature and role of these cells in human disease.

The ontogeny of the porcine immune system

Cellular and humoral aspects of the immune response develop sequentially in the fetus. During the ontogeny, the pluripotent stem cells emerge and differentiate into all hematopoietic lineages. Basic questions including the identification of the first lympho-hematopoietic sites, the origin of T and B lymphocytes, the development of different subpopulations of alphabeta T, gammadelta T and B lymphocytes as well as development of innate immunity and the acquisition of full immunological capacities are discussed here for swine and compared with other species. The description of related topics such as fertilization, morphogenesis, maternal-fetal-neonatal physiology and early neonatal development are also discussed.

Immune-Cell Lineage Commitment: Translation from Mice to Humans

Much of the current understanding in hematopoietic stem cell differentiation into immune-cell lineages comes from mouse studies, but how well does it translate to the human system?