Porcine signal regulatory protein alpha binds to human CD47 to inhibit phagocytosis: Implications for human hematopoietic stem cell transplantation into severe combined immunodeficient pigs

BACKGROUND

Severe combined immunodeficient (SCID) pigs are an emerging animal model being developed for biomedical and regenerative medicine research. SCID pigs can successfully engraft human-induced pluripotent stem cells and cancer cell lines. The development of a humanized SCID pig through xenotransplantation of human hematopoietic stem cells (HSCs) would be a further demonstration of the value of such a large animal SCID model. Xenotransplantation success with HSCs into non-obese diabetic (NOD)-derived SCID mice is dependent on the ability of NOD mouse signal regulatory protein alpha (SIRPA) to bind human CD47, inducing higher phagocytic tolerance than other mouse strains. Therefore, we investigated whether porcine SIRPA binds human CD47 in the context of developing a humanized SCID pig.

METHODS

Peripheral blood mononuclear cells (PBMCs) were collected from SCID and non-SCID pigs. Flow cytometry was used to assess whether porcine monocytes could bind to human CD47. Porcine monocytes were isolated from PBMCs and were subjected to phagocytosis assays with pig, human, and mouse red blood cell (RBC) targets. Blocking phagocytosis assays were performed by incubating human RBCs with anti-human CD47 blocking antibody B6H12, non-blocking antibody 2D3, and nonspecific IgG1 antibody and exposing to human or porcine monocytes.

RESULTS

We found that porcine SIRPA binds to human CD47 in vitro by flow cytometric assays. Additionally, phagocytosis assays were performed, and we found that porcine monocytes phagocytose human and porcine RBCs at significantly lower levels than mouse RBCs. When human RBCs were preincubated with CD47 antibodies B6H12 or 2D3, phagocytosis was induced only after B6H12 incubation, indicating the lower phagocytic activity of porcine monocytes with human cells requires interaction between porcine SIRPA and human CD47.

CONCLUSIONS

We have shown the first evidence that porcine monocytes can bind to human CD47 and are phagocytically tolerant to human cells, suggesting that porcine SCID models have the potential to support engraftment of human HSCs.

Creating effective biocontainment facilities and maintenance protocols for raising specific pathogen-free, severe combined immunodeficient (SCID) pigs

Severe combined immunodeficiency (SCID) is defined by the lack of an adaptive immune system. Mutations causing SCID are found naturally in humans, mice, horses, dogs, and recently in pigs, with the serendipitous discovery of the Iowa State University SCID pigs. As research models, SCID animals are naturally tolerant of xenotransplantation and offer valuable insight into research areas such as regenerative medicine, cancer therapy, as well as immune cell signaling mechanisms. Large-animal biomedical models, particularly pigs, are increasingly essential to advance the efficacy and safety of novel regenerative therapies on human disease. Thus, there is a need to create practical approaches to maintain hygienic severe immunocompromised porcine models for exploratory medical research. Such research often requires stable genetic lines for replication and survival of healthy SCID animals for months post-treatment. A further hurdle in the development of the ISU SCID pig as a biomedical model involved the establishment of facilities and protocols necessary to obtain clean SPF piglets from the conventional pig farm on which they were discovered. A colony of homozygous SCID boars and SPF carrier sows has been created and maintained through selective breeding, bone marrow transplants, innovative husbandry techniques, and the development of biocontainment facilities.

The effect of pegylated granulocyte colony-stimulating factor treatment prior to experimental mastitis in lactating Holsteins

Neutrophils are the first-acting and most prominent cellular defense against mastitis-causing pathogens. This makes neutrophil activation and expansion obvious candidates for targeted therapeutics. The granulocyte colony-stimulating factor (G-CSF) cytokine stimulates the bone marrow to produce granulocytes and stem cells and release them into the bloodstream, which results in neutrophilia as well as increasing the presence of other progenitor cells in the bloodstream. A pegylated form of G-CSF (PEG-gCSF) has been shown to significantly decrease naturally occurring mastitis rates in cows postpartum. The use of PEG-gCSF had not been evaluated in response to an experimental mastitis challenge. In an effort to examine the effect and mechanism of PEG-gCSF treatment, we challenged 11 mid-lactation Holsteins with ∼400 cfu Escherichia coli P4 by intramammary infusion. Five cows received 2 PEG-gCSF injections, one at 14 d and the other at 7 d before disease challenge, and 6 cows remained untreated. To evaluate the response of cows to the PEG-gCSF treatment, we measured complete blood counts, somatic cell counts, bacterial counts, milk yield, and feed intake data. The PEG-gCSF-treated cows had significantly increased circulating levels of neutrophils and lymphocytes after each PEG-gCSF injection, as well as following mastitis challenge. The PEG-gCSF-treated cows had significantly lower bacterial counts and lower milk BSA levels at the peak of infection. In addition, control cows had significant decreases in milk yield postinfection and significantly reduced feed intake postinfection compared with PEG-gCSF-treated cows. Collectively, PEG-gCSF treatment resulted in reduced disease severity when administered before a bacterial challenge. Mechanistically, we show that G-CSF treatment increases cell surface expression of an E-selectin ligand before infection on neutrophils and monocytes found in the blood. These cells quickly disappear from the blood shortly after infection, suggesting a mechanism for the reduced mastitis severity by priming immune cells for quick targeting to the site of infection.

Porcine SIRPA binds to human CD47 to inhibit phagocytosis: Implications for human hematopoietic stem cell transplantation into severe combined immunodeficient (SCID) pigs

Xenotransplantation success with human hematopoietic stem cells into non-obese diabetic (NOD)-derived severe combined immunodeficient (SCID) mice is dependent on the ability of NOD mouse signal regulatory protein alpha (SIRPA) to bind human CD47 (hCD47). Interaction between NOD mouse SIRPA and hCD47 results in higher phagocytic tolerance compared to other SCID mouse strains. In the context of developing a humanized SCID pig, we investigated whether porcine SIRPA binds hCD47 and subsequently inhibit phagocytosis of human cells. Porcine SIRPA bound to hCD47 in vitro. Porcine monocytes phagocytosed human and porcine red blood cells (RBCs) at significantly lower levels than mouse RBCs. When human RBCs were pre-incubated with CD47 blocking antibody B6H12 or non-blocking antibody 2D3, phagocytosis was induced only after B6H12 incubation, indicating the lower phagocytic activity requires interaction between porcine SIRPA and hCD47. Collectively, we provide the first evidence that porcine monocytes may be phagocytically tolerant towards human cells, suggesting that porcine SCID models have the potential to support engraftment of human stem cells.

Investigating the effect of pegylated granulocyte colony stimulating factor therapy on experimental mastitis in lactating Holsteins

Mastitis dominates dairy economic and health concerns. Neutrophils are the most prominent cellular defense against mastitis causing pathogens making neutrophil expansion and activation, prime candidates for targeted therapeutics. The granulocyte colony-stimulating factor (gCSF) cytokine targets neutrophil differentiation and causes neutrophilia. Recently pegylated (PEG) gCSF has been shown to decrease naturally-occurring cases of mastitis. To better understand the mechanisms employed by PEG-gCSF to reduces mastitis incidence we have evaluated PEG-gCSF in an experimental mastitis challenge. We challenged 11 lactating Holsteins with Escherichia coli by intra-mammary infusion. Cows received injections of PEG-gCSF 14 days and 7 days, prior to disease challenge. While somatic cell counts in the milk did not differ between treated and non-treated animals, the treated cows had lower bacterial counts in the 48 hours post infection (P < 0.05). Bovine serum albumin levels in milk, a measure of disease severity, were also lower 24 hours post challenge for PEG-gCSF treated animals. Post challenge, control cattle had reduced milk yield (P = 0.05) and a lower percent feed intake (P < 0.01) compared to treated cows. PEG-gCSF treated cattle significantly increased circulating levels of neutrophils (P < 0.01), as well as monocyte and lymphocyte responses (P < 0.05). We demonstrated that PEG-gCSF treated cattle had reduced disease severity and have begun to further elucidate the mechanism of neutrophil immune responses. This data suggests the utilization of PEG-gCSF as a potential antibiotic alternative to mastitis treatment and prevention and supports neutrophil focused therapies as an important facet of mastitis disease protection.

T Cell Lymphoma and Leukemia in Severe Combined Immunodeficiency Pigs following Bone Marrow Transplantation: A Case Report

After the discovery of naturally occurring severe combined immunodeficiency (SCID) within a selection line of pigs at Iowa State University, we found two causative mutations in the Artemis gene: haplotype 12 (ART12) and haplotype 16 (ART16). Bone marrow transplants (BMTs) were performed to create genetically SCID and phenotypically immunocompetent breeding animals to establish a SCID colony for further characterization and research utilization. Of nine original BMT transfer recipients, only four achieved successful engraftment. At approximately 11 months of age, both animals homozygous for the ART16 mutation were diagnosed with T cell lymphoma. One of these ART16/ART16 recipients was a male who received a transplant from a female sibling; the tumors in this recipient consist primarily of Y chromosome-positive cells. The other ART16/ART16 animal also presented with leukemia in addition to T cell lymphoma, while one of the ART12/ART16 compound heterozygote recipients presented with a nephroblastoma at a similar age. Human Artemis SCID patients have reported cases of lymphoma associated with a "leaky" Artemis phenotype. The naturally occurring Artemis SCID pig offers a large animal model more similar to human SCID patients and may offer a naturally occurring cancer model and provides a valuable platform for therapy development.

SCID pigs: An emerging large animal NK model

Severe Combined ImmunoDeficiency (SCID) is defined as the lack or impairment of an adaptive immune system. Although SCID phenotypes are characteristically absent of T and B cells, many such SCID cellular profiles include the presence of NK cells. In human SCID patients, functional NK cells may impact the engraftment success of life saving procedures such as bone marrow transplantation. However, in animal models, a T cell-, B cell-, NK cell+ environment provides a valuable tool for asking specific questions about the extent of the innate immune system function as well as emerging NK targeted therapies against cancer. Physiologically and immunologically the pig is more similar to the human than common rodent research animals. This review discusses why the T- B- NK+ SCID pig may offer a more relevant model for development of human SCID patient therapies as well as provide an opportunity for systematic exploration of the role of NK cells in artiodactyl immunity.

NK cells are intrinsically functional in pigs with Severe Combined Immunodeficiency (SCID) caused by spontaneous mutations in the Artemis gene

We have identified Severe Combined Immunodeficiency (SCID) in a line of Yorkshire pigs at Iowa State University. These SCID pigs lack B-cells and T-cells, but possess Natural Killer (NK) cells. This SCID phenotype is caused by recessive mutations in the Artemis gene. Interestingly, two human tumor cell lines, PANC-1 and A375-SM, survived after injection into these SCID pigs, but, as we demonstrate here, these cells, as well as K562 tumor cells, can be lysed in vitro by NK cells from SCID and non-SCID pigs. NK cells from both SCID and non-SCID pigs required activation in vitro with either recombinant human IL-2 or the combination of recombinant porcine IL-12 and IL-18 to kill tumor targets. We also showed that SCID NK cells could be activated to produce perforin, and perforin production was greatly enhanced in NK cells from both SCID and non-SCID pigs after IL-2 cytokine treatment. While CD16+, CD172- NK cells constituted an average of only 4% in non-SCID pigs, NK cells averaged 27% of the peripheral blood mononuclear cell population in SCID pigs. We found no significant differences in killing activity per NK cell between SCID and non-SCID pigs. We conclude that survival of human cancer cells in these SCID pigs is not due to an intrinsic defect in NK cell killing ability.

Racial variation in spontaneous fetal deaths at 20 weeks or older in upstate New York, 1980-86

The distribution of spontaneous fetal deaths (at age 20 weeks or more) by maternal race has received considerably less study than other adverse pregnancy outcomes. The purpose of this study was twofold--(a) to describe spontaneous fetal deaths among white, black, and American Indian women and (b) to determine if there was any variation by International Classification of Diseases, Ninth Revision (ICD-9) cause of death, gestational age at death, or maternal age at loss among these groups of mothers. Using the fetal death certificate registry maintained by the New York State Department of Health, 8,592 spontaneous fetal deaths at age 20 weeks or more were identified among upstate (exclusive of New York City) mothers between 1980 and 1986. By race it was 7,300 for white women, 1,257 for black women, and 27 for American Indian women. Spontaneous fetal death rates varied by maternal race as listed on vital records--black, 13.5 per 1,000 total births, white, 8.3, and American Indian, 8.1. The three leading causes of death (ICD-9,779, 762, and 761) did not vary by maternal race. Gestational age at death, imputed from last menstrual period, did vary by maternal race. Fetal deaths to white and black mothers were observed to occur most often between 24 weeks of pregnancy (39 percent) and 32 weeks (43 percent), while American Indian fetal deaths generally occurred later (more than 33 weeks) in pregnancy (41 percent). Most spontaneous fetal deaths occurred to mothers ages 20-29 regardless of race. Black teenage mothers, however, experienced the largest proportion of losses(23 percent) compared with white (10 percent) and American Indian (I I percent) teenage mothers.

Comparison of Native American births in upstate New York with other race births, 1980-86

The purpose of this study was to describe the neonatal characteristics of Native American (Indian) infants and the antenatal characteristics of their mothers as compared with white, black, and other race infants. The study population comprised 979,444 live births to upstate New York (exclusive of New York City) resident mothers between 1980 and 1986. Data were abstracted from vital records (birth certificates) and analyzed using a variety of descriptive statistics. Mothers of Native American and black infants had similar antenatal profiles (that is, younger, higher parity, lower educational attainment, and delayed initiation of prenatal care), which differed from mothers of white or other race infants. Despite having at-risk mothers, Native American infants were similar to white and other race infants with respect to the percentage of births that were considered low birth weight or premature. Black infants were twice as likely as the other three groups of infants to be low birth weight or premature. These findings suggest that other factors appear to be important in determining neonatal outcome and that typical at-risk antenatal profile of mothers may not be consistent across all racial groups.