Naturally developing memory T cell xenoreactivity to swine antigens in human peripheral blood lymphocytes

Role of human and porcine MHC DRB1 alleles in determining the intensity of individual human anti-pig T-cell responses

BACKGROUND

Differences in quality and strength of immune responses between individuals are mainly due to polymorphisms in major histocompatibility complex (MHC) molecules. Focusing on MHC class-II, we asked whether the intensity of human anti-pig T-cell responses is influenced by genetic variability in the human HLA-DRB1 and/or the porcine SLA-DRB1 locus.

METHODS

ELISpot assays were performed using peripheral blood mononuclear cells (PBMCs) from 62 HLA-DRB1-typed blood donors as responder and the porcine B cell line L23 as stimulator cells. Based on the frequency of IFN-γ-secreting cells, groups of weak, medium, and strong responder individuals were defined. Mixed lymphocyte reaction (MLR) assays were performed to study the stimulatory capacity of porcine PBMCs expressing different SLA-DRB1 alleles.

RESULTS

Concerning the MHC class-II configuration of human cells, we found a significant overrepresentation of HLA-DRB1*01 alleles in the medium/strong responder group as compared to individuals showing weak responses to stimulation with L23 cells. Evaluation of the role of MHC class-II variability in porcine stimulators revealed that cells expressing SLA-DRB1*06 alleles triggered strong proliferation in approximately 70% of humans. Comparison of amino acid sequences indicated that strong human anti-pig reactivity may be associated with a high rate of similarity between human and pig HLA/SLA-DRB1 alleles.

CONCLUSION

Variability in human and porcine MHC determines the intensity of individual human anti-pig T-cell responses. MHC typing and cross-matching of prospective recipients of xenografts and donor pigs could be relevant to select for donor-recipient combinations with minimal anti-porcine immunity.

Porcine antigen-specific IFN-γ ELISpot as a potentially valuable tool for monitoring cellular immune responses in pig-to-non-human primate islet xenotransplantation

BACKGROUND

Recent progress in xenotransplantation of porcine islets to non-human primates (NHPs) gives hope for human clinical trials in the near future. Thus, implementation of an appropriate monitoring method to detect the development of detrimental porcine antigen-specific cellular immune responses is necessary. The enzyme-linked immunospot (ELISpot) assay has been widely used to monitor antigen-specific alloreactive T-cell responses in humans; however, the utility of porcine islet-specific ELISpot assay has not yet been thoroughly evaluated for pig-to-NHPs intraportal islet xenotransplantation.

METHODS

The optimal ELISpot assay conditions, including the number of responder and stimulator cells and the provision of costimulation, were determined. Then, ELISpot assays were conducted on serial stocks of peripheral blood mononuclear cell (PBMC) samples previously isolated from NHP recipients transplanted with porcine islets. Either splenocytes from donor pigs or pancreatic islets from third-party pigs were used for antigen stimulation. At the same time, the ratio of CD4(+) /CD8(+) T cells and the percentage of CD4(+) FoxP3(+) T cells in the peripheral blood were evaluated. Finally, liver biopsy samples were evaluated to assess the immunopathology of the grafts.

RESULTS

The optimal conditions for the ELISpot assay were defined as 2.5 × 10(5) responder cells incubated with 5.0 × 10(5) stimulator cells in 96-well, flat-bottom plates without further costimulation. Using donor splenocytes as stimulators, a serial interferon-gamma (IFN-γ) ELISpot assay with PBMCs from the monkeys with prolonged porcine islet grafts (>180 days) demonstrated that the number of donor antigen-specific IFN-γ-producing cells significantly increased upon overt graft rejection. However, use of third-party porcine islets as stimulators did not reflect graft rejection, suggesting that the use of donor-specific PBMCs, and not tissue (porcine islet)-specific cells, as stimulators could better serve the purpose of this assay in adult porcine islet transplantation. IFN-γ spot number was neither influenced by the peripheral blood CD4(+) /CD8(+) T-cell ratio nor the percentage of CD4(+) FoxP3(+) T cells. Finally, in cases of overt graft rejection, the number of IFN-γ spots and the graft-infiltrating T cells in biopsied liver samples increased simultaneously.

CONCLUSION

Use of PBMCs in a porcine antigen-specific IFN-γ ELISpot assay is a reliable method for monitoring T-cell-mediated rejection in pig-to-NHP islet xenotransplantation.

Is there a correlation between anti-pig antibody levels in humans and geographic location during childhood?

BACKGROUND

An initial observation suggested high levels of anti-pig antibodies in healthy humans who had spent their childhood in the Middle East. We tested larger cohorts to determine whether anti-pig antibody levels correlated with the geographic location in which the subject spent his/her childhood, because this might have implications for clinical trials of xenotransplantation.

METHODS

Anti-pig IgM and IgG levels (by flow cytometry using peripheral blood mononuclear cells from wild-type and α1,3-galactosyltransferase gene-knockout pigs) and anti-Gal IgM and IgG levels (by enzyme-linked immunosorbent assay) were measured in 75 volunteers. Comparisons of antibody levels were also made based on subject age, gender, ABO blood group, diet, and history of vaccination.

RESULTS

Antibody binding to α1,3-galactosyltransferase gene-knockout pig cells was less than to wild-type cells. There was a reduction in anti-pig IgM and anti-Gal IgM, but a slight increase in anti-nonGal IgG, with age. Women had higher levels of anti-Gal IgM than men. Blood group A subjects had higher levels of anti-pig IgM and IgG than those of group AB. Diet had no influence on antibody levels. Typhoid or measles-mumps-rubella vaccination was associated with lower anti-nonGal IgG or anti-Gal IgG, respectively, whereas influenza vaccination was associated with higher anti-nonGal IgG. There were some significant variations in antibody levels associated with location during childhood, with subjects from the Middle East demonstrating higher anti-nonGal IgG and anti-Gal IgG.

CONCLUSION

Clinical trials of xenotransplantation may be influenced by various factors, including the geographic location of the recipient during childhood, possibly associated with exposure to different microorganisms.

Suppressive efficacy and proliferative capacity of human regulatory T cells in allogeneic and xenogeneic responses

BACKGROUND

An understanding of the mechanisms that suppress the human anti-pig cellular response is key for xenotransplantation. We have compared the ability of human regulatory T cells (Tregs) to suppress xenogeneic and allogeneic responses in vitro.

METHODS

Human peripheral blood mononuclear cells (PBMC), CD4+ T cells, or CD4+ CD25- T cells were stimulated with irradiated human or wild type (WT) or alpha1,3-galactosyltransferase gene-knockout (GT-KO) pig PBMC in the presence or absence of human CD4+ CD25 high Tregs. In separate experiments, 5- (and 6)-carboxyfluorescein diacetate succinimidyl ester-labeled human CD4+ T cells were stimulated with human or pig PBMC. The expansion and precursor frequencies of allo- and xenoreactive Tregs were assessed by labeling with FoxP3 mAb and flow cytometric analysis.

RESULTS

The responses of human PBMC, CD4+ T cells, and CD4+ CD25- T cells to pig PBMC were stronger than to human PBMC (P<0.05). Human anti-GT-KO responses were weaker than anti-WT responses (P<0.05). Human CD4+ CD25 high Tregs suppressed proliferation of CD4+ CD25- T cells to both human and pig PBMC stimulator cells with the same efficiency. Alloreactive CD4+ CD25+ FoxP3 high responder T cells proliferated more than their xenoreactive counterparts (P<0.05), although xenoreactive CD4+ CD25+ T cells proliferated more than alloreactive cells (P<0.05). There was no difference in precursor frequency between allo- and xeno-reactive CD4+ CD25+ FoxP3 high cells.

CONCLUSIONS

Human T-cell responses to pig cells are stronger than to allogeneic cells. The human response to GT-KO PBMC is weaker than to WT PBMC. Although human Tregs can suppress both responses, expansion of CD4+ CD25+ FoxP3 high cells against pig PBMC is weaker than against human PBMC. More human Tregs may be required to suppress the stronger xenogeneic response.

Essential role of sphingosine-1-phosphate receptor 1-bearing CD8+CD44+CCR7+ T cells in acute skin allograft rejection

A subset of naturally formed sphingosine-1-phosphate receptor 1 (S1P1)-bearing CD8(+)CD44(+)CCR7(+) memory T cells has been identified in transplant recipient BALB/c (H-2(d)) mice. The frequency of this subset of memory T cells is significantly increased in the spleen, lymph nodes and skin grafts in the recipient BALB/c mice during acute skin allograft rejections. The immune-reconstitution with CD8(+)CD44(+)CCR7(+)S1P1(+) memory T cells facilitates acute skin allograft rejection in SCID mice. Being Th1-polarized and cytotoxic, CD8(+)CD44(+)CCR7(+)S1P1(+) memory T cells proliferate and differentiate immediately into effectors upon encountering allo-antigens. A siRNA against S1P1 inhibits CD8(+)CD44(+)CCR7(+)S1P1(+) memory T cell-mediated acute skin allograft rejection in SCID mice by means of knocking-down S1P1-expression. CCL21 mutant (CCL21-DeltaCT) has been used to compete with wild-type CCL21 in the course of binding to CCR7. Combined administration of siRNA S1P1 and CCL21-DeltaCT significantly prolongs the survival of skin allograft in the recipient BALB/c mice by means of inhibiting accumulation of CD8(+)CD44(+)CCR7(+)S1P1(+) memory T cells in the spleen and the skin grafts. Our data provide direct evidence that S1P1 and CCR7 are involved in the proliferation and trafficking of CD8(+)CD44(+)CCR7(+)S1P1(+) memory T cells. S1P1 may serve as a functional marker for CD8(+)CD44(+)CCR7(+) memory T cells. Targeting CD8(+)CD44(+)CCR7(+)S1P1(+) T cells may be a useful strategy to prolong the survival of allograft transplant.

Deviation of xenogeneic immune response and bystander suppression in rats fed porcine blood mononuclear cells

Reducing or deviating xenogeneic immune response prior to xenotransplantation may enhance the efficacy of conventional immunosuppressive therapies in prolonging xenograft survival. The potential to suppress or steer immune responses by oral administration of xenoantigens was evaluated. Based on knowledge of oral tolerance, hypotheses tested were that feeding xenoantigens would inhibit cell-mediated immune response (CMIR) and production of antibodies associated with graft rejection and induce bystander suppression. DA and LEW rats, high and low responders to xenoantigens, respectively, were fed dead porcine blood mononuclear cells (PBMC) and subsequently received live PBMC and hen egg-white lysozyme (HEWL, a third-party antigen) by subcutaneous injection. Delayed-type hypersensitivity (DTH) to PBMC was an indicator of CMIR. Quantification of T(H)1 (IgG(2b)) and T(H)2 (IgG(1))-associated antibodies and their ratio measured magnitude and bias of the antibody-mediated response to PBMC and HEWL. Feeding PBMC reduced IgG(2b) antibody production by 90% (DA) and 71% (LEW) and increased IgG(1) antibodies by 116% in DA but not LEW rats (p<or=0.05). DTH was unaffected (DA) or increased (LEW) in antigen-fed rats. Bystander suppression was demonstrated by inhibition of antibodies to HEWL in DA rats. LEW rats did not respond to HEWL. Evaluation of tertiary immune response to PBMC revealed DTH and antibodies to xenoantigens was inhibited. These data demonstrated orally-induced immune deviation and bystander suppression in rats given PBMC that may favor discordant xenograft survival and suggests a method to suppress xenogeneic immunity in high responders by repeated immunization with xenoantigens.

T-cell responses during pig-to-primate xenotransplantation

Xenotransplantation using porcine organs may resolve a chronic shortage of donor organs for clinical transplantation if significant immunological barriers can be overcome. To determine the potential role of T lymphocytes in Xenograft (Xg) rejection, we transplanted transgenic hCD46 porcine hearts heterotopically into baboon recipients.

METHODS

Recipients were treated to deplete anti-Gal antibody with a non-antigenic alpha-Gal polyethylene glycol polymer (TPC) (n = 2), TPC plus rituximab (anti-CD20) (n = 1) or were untreated (n = 1). None of the recipients received T-cell immunosuppression.

RESULTS

All Xgs failed within 7 days and showed evidence of a mixed humoral and cellular rejection process. Cellular infiltration consisting primarily of CD4+ T cells and few CD8+ T cells. Proliferation and cytotoxicity assays showed sensitization of CD4+ and CD8+ T cells that reacted with porcine IFN-gamma (pIFN-gamma)-stimulated porcine aortic endothelial cells (PAEC). The CD4+ lymphocytes displayed greater cytotoxicity than CD8+ cells. An increased frequency of PAEC-specific interleukin (IL) 2 and IFN-gamma-secreting T cells was observed, suggesting a Th1 cytokine bias. An increase in the percentage of circulating CD4+CD28- cells was observed at the time of rejection and over 50% of the CD4+ cells recovered from residual pig tissue at necropsy lacked CD28 expression.

CONCLUSIONS

These findings show that lymphocytes are efficiently stimulated by PAEC antigens and can mediate direct tissue destruction. These studies (1) provide an insight into the potential of cellular-mediated cardiac Xg rejection, (2) show for the first time the induction of cytotoxic pig-specific CD4+CD28- lymphocytes and (3) provide a rational basis for determining different modes of immunosuppression to treat Xg rejection.

Human T lymphocyte responses against lung cancer induced by recombinant truncated mouse EGFR

The induction of active cellular responses against EGFR should be a promising approach for the treatment of those receptor-positive tumors. However, the immunity against EGFR is presumably difficult to elicit by vaccine based on self or syngeneic EGFR due to the immune tolerance acquired during the development in immune system. We proposed a model to break immune tolerance against self-EGFR through an altered immunogen source based on xenogeneic homologous EGFR. We have previously shown human EGFR as a xenoantigen could induce specific immune responses in mouse and cross-react with mouse EGFR, and resulted in therapeutic benefits for EGFR-positive mouse tumor. Here, we show a recombinant form of extracellular domain of mouse EGFR, in the presence of DCs, could activate human peripheral T cells to proliferate, secret IFN-gamma, the induced responses could cross-react with human EGFR and kill autologous EGFR-positive lung cancer cells which could be blocked by anti-CD8 and anti-MHC class I antibody. There is no detectable cytotoxical activity against lung tissue, liver tissue and kidney tissue derived from paracancerous normal tissue. These observations suggest that antitumor immunity induced by the truncated mouse EGFR may be provoked in a cross-reaction between mouse EGFR and self-EGFR, and may provide insight into treatment of EGFR-positive tumors through induction of the autoimmune responses against EGFR.

Direct Antigen Presentation by a Xenograft Induces Immunity Independently of Secondary Lymphoid Organs

The location of immune activation is controversial during acute allograft rejection and unknown in xenotransplantation. To determine where immune activation to a xenograft occurs, we examined whether splenectomized alymphoplastic mice that possess no secondary lymphoid organs can reject porcine skin xenografts. Our results show that these mice rejected their xenografts, in a T cell-dependent fashion, at the same tempo as wild-type recipients, demonstrating that xenograft rejection is not critically dependent on secondary lymphoid organs. Furthermore, we provide evidence that immune activation in the bone marrow did not take place during xenograft rejection. Importantly, immunity to xenoantigens was only induced after xenotransplantation and not by immunization with porcine spleen cells, as xenografted mutant mice developed an effector response, whereas mutant mice immunized by porcine spleen cells via i.p. injection failed to do so. Moreover, we provide evidence that antixenograft immunity occurred via direct and indirect Ag presentation, as recipient T cells could be stimulated by either donor spleen cells or recipient APCs. Thus, our data provide evidence that direct and indirect Ag presentation by a xenograft induces immunity in the absence of secondary lymphoid organs. These results have important implications for developing relevant xenotransplantation protocols.

CD28 costimulation enhances the sensitivity of the ELISPOT assay for detection of antigen-specific memory effector CD4 and CD8 cell populations in human diseases

The frequencies of antigen-specific memory cells are often low in chronic disease states related to infection and autoimmunity, making detection of such populations difficult, even with high sensitivity assays such as the cytokine enzyme-linked immunospot (ELISPOT). The spectrum and function of antigen presenting cells (APC) in the peripheral compartment can differ considerably from the inflamed target organ. In order to approximate the costimulatory environment of the target organ, we measured T cell responses with and without the addition of agonistic anti-CD28 antibody in the ELISPOT assay. CD4 and CD8 IFN-gamma responses to viral (hepatitis C) and autoimmune antigens (islet cell) were tested in 10 hepatitis C and 8 type 1 diabetic as well as healthy control subjects. IFN-gamma responses to tetanus toxoid, mumps and cytomegalovirus (CMV) protein antigen, as well as Epstein-Barr virus and CMV peptides were also measured in healthy control subjects. We found higher frequencies of T cells reactive to protein and peptide antigens when anti-CD28 antibody was present, often detecting responses only in the presence of anti-CD28 antibody. These results demonstrate that anti-CD28 antibody signal enhanced ELISPOT assays can facilitate the identification of low precursor frequency T cells in chronic infectious and autoimmune disease states where suboptimal costimulatory environment may exist in the periphery. The use of such costimulation may also enable a more quantitative assessment of circulating memory effector T cell frequency.

Xenogeneic cells and superantigen induce human T-cell activation in the absence of T-cell recognition of xenoantigen

The extent of interaction between human T-lymphocytes and xenogeneic antigen-presenting cells (APCs) is species-dependent. A successful interaction requires high-affinity receptor-ligand pairing across species and recognition of antigens presented by foreign major histocompatibility complex (MHC). A deficient human T-lymphocyte response to xenogeneic cells is likely the result of a defect in these interactions. However, the requirements for a T-cell response to superantigen (SAg) may differ from those of other T-cell responses. Using irradiated murine splenocytes, which are believed to be incapable of eliciting human T-cell responses, and porcine aortic endothelial cells (PAECs) as the APC populations, we studied the human T-lymphocyte response to antigens presented by these cells. Direct proliferation of human T-lymphocytes to SAg presented by murine APCs was demonstrated; it was blocked by anti-human LFA-1 and anti-murine MHC class II but not by anti-human MHC class II. PAECs also presented SAg to human T-cells, generating a proliferative response greater than the primary response to porcine xenoantigen. Culture of human T-cells with murine splenocytes or PAECs and SAg Staphylococcus enterotoxin A (SEA) for 7 days primed human T-cells to proliferate in a secondary culture in response to autologous APCs. This autologous secondary response was human MHC class II-dependent and was inhibited by anti-human LFA-1, anti-human CD2, and anti-human CD98. Surprisingly, both of these responses were also blocked by anti-SEA, suggesting that despite vigorous washing, a small amount of functionally important SAg was carried over from primary to secondary culture, probably bound to the surface of T-cells. Xenogeneic APCs, even those that fail to stimulate human T-cells directly, can serve as APCs for primary human T-cell responses. After such interactions T-cells can develop secondary responses in autologous interactions based on retention of minute amounts of SAg. Such interactions may have important implications for xenotransplantation.

Exploiting virus stealth technology for xenotransplantation: reduced human T cell responses to porcine cells expressing herpes simplex virus ICP47

Direct recognition of porcine major histocompatibility complex (MHC) proteins by human T cells is well documented. Eliminating donor (porcine) MHC proteins may therefore be beneficial in pig-to-human xenotransplants. To this end, we have attempted to exploit viral stealth mechanisms to eliminate pig MHC class I cell-surface expression. PK(15) (pig kidney) cells stably transfected with the herpes simplex virus (HSV) ICP47 gene [PK(15)-ICP47 cells] exhibited a dramatic reduction of MHC class I cell-surface expression when compared with untransfected PK(15) cells. To test the effect of down-regulation of porcine MHC class I on human cellular immune responses, a human CD8+ enriched T cell line (anti-PK15 T cells) with reactivity towards PK(15) cells was derived by repeated stimulation of human T cells with PK(15) cells stably transfected with the costimulatory molecule B7.1 [PK(15)-B7.1 cells]. Anti-PK15 T cells efficiently lyzed PK(15) cells but not PK(15)-ICP47 (class I negative) cells. Consistent with effector function, anti-PK15 T cells showed a robust proliferative response to PK(15)-B7.1 cells but did not proliferate at all to PK(15)-B7.1 cells which also expressed HSV ICP47. These results suggest that virus stealth technology can be exploited for xenotransplantation.

Direct killing of xenograft cells by CD8+ T cells of discordant xenograft recipients

BACKGROUND

Long-term pig xenografts in monkeys demonstrated the infiltration of CD8 T cells into pig cartilage xenografts, transplanted into monkeys. The objective of the present study was to determine in an experimental animal model whether CD8 T cells in pig xenograft recipients exert any direct cytotoxic effect on pig cells.

METHODS

The killing of xenograft cells by CD8 T cells, obtained from xenograft recipients, was studied in alpha1,3galactosyltransferase knockout mice that were repeatedly injected intraperitoneally with pig kidney membranes. The pig kidney cell line PK15, which shares many antigens with pig kidney membranes, served as a model for xenograft target cells in cytotoxicity assays. Cell lines from other species were also studied as target cells.

RESULTS

Lymphocytes obtained freshly from spleens of mice immunized with pig kidney membranes failed to display significant cytotoxic activity against pig cells. However, incubation of these lymphocytes with irradiated PK15 cells and addition of recombinant interleukin (IL)-2 (100 U/mL), on the third day of incubation, resulted in extensive proliferation and expansion of CD8 cytotoxic T lymphocytes (CTL). These CTL, obtained after 12 days of incubation, killed nonspecifically pig, human, and mouse normal and malignant cells. These CTL were not generated in cultures in the absence of stimulatory pig cells or in the absence of IL-2. These CTL could not be generated in cultures of lymphocytes from naive mice that were incubated with PK15 cells and IL-2.

CONCLUSIONS

The data obtained imply that CD8 T cells from xenograft recipients can be stimulated in vitro by xenoantigens and IL-2 to differentiate into highly reactive nonspecific CTL that are capable of killing a large variety of xenogeneic and syngeneic cells. Similar in vivo microenvironmental conditions within the xenograft may induce the local differentiation of infiltrating CD8 T cells into CTL that can destroy nonspecifically adjacent xenograft cells. Such cells may not be active outside the xenograft because of the absence of IL-2 in sufficiently high concentrations.

Comprehensive determinant mapping of the hepatitis C-specific CD8 cell repertoire reveals unpredicted immune hierarchy

While CD8 cells are thought to play an important role in the control hepatitis C infection, low frequencies of virus-specific cells and high numbers of potential determinants have made it challenging to obtain direct and comprehensive data regarding fine specificity and clonal size of the CD8 cells involved. Most assays suited for measuring CD8 cell frequencies require prior knowledge of immune dominant peptides. While there are excellent algorithms for predicting MHC-peptide binding strength for particular class I alleles, it is unknown how accurate these algorithms are in predicting the actual determinant recognized in an individual coexpressing other class I alleles. We used a high throughput ELISPOT approach to test for responses to every possible 9-mer determinant within the 191 residue hepatitis C core protein in addition to 61 previously defined CD8 cell determinants. The amino acid sequence of each determinant recognized was compared with HLA-binding predictions for the expressed class I alleles. These data show feasibility for and importance of comprehensive direct ex vivo monitoring, an approach which should facilitate design of antiviral immunotherapeutic strategies.

ELISPOT analysis of hepatitis C virus protein-specific IFN-gamma-producing peripheral blood lymphocytes in infected humans with and without cirrhosis

An improved ability to monitor hepatitis C virus (HCV)-specific T cell immunity in infected patients may provide novel information regarding the pathogenesis and prognosis of this infection. We used an ELISPOT assay to analyze a cross-section of HCV-infected humans. HCV-infected patients without cirrhosis, those with cirrhosis, and controls with other liver diseases were tested for recall responses to HCV Core and NS3 proteins. Peripheral blood lymphocytes (PBLs) from HCV-infected patients without cirrhosis responded to NS3 and Core proteins, producing predominantly IFN-gamma, with little IL-4 or IL-5. In contrast, PBLs from HCV-infected patients with cirrhosis responded to NS3, but not to the Core protein, suggesting a selectively altered immune state during cirrhosis. Our data provide support for the notion that HCV-specific IFN-gamma-producing immunity is important in the pathogenesis of progressing HCV-related disease.

Sequence of the swine major histocompatibility complex region containing all non-classical class I genes

A segment of 158,063 nucleotides of the pig major histocompatibility complex (SLA) and corresponding to the junction of the class I and class III regions was sequenced entirely. The centromeric part of the segment contained six class III genes including the three tumor necrosis factor genes, while the telomeric part contained three genes belonging to the class I region. The order and the molecular organization of these genes were exactly conserved in the SLA and HLA complexes, except for the SC1 gene which displayed a shift of the reading frame in swine. The cluster of the three SLA class I-related genes (Ib) and the MIC1 and MIC2 genes were located in the middle of the segment, in the following order from the centromeric side onwards, SLA-6, SLA-7, SLA-8, MIC-1 and MIC-2. All three SLA Ib genes displayed an overall molecular structure compatible with the expression of membrane-anchored glycoproteins. The SLA-7 and SLA-8 genes bear greater resemblance than to the SLA-6 gene. Six SLA-6 alleles have been previously defined differing each from the other by unique point mutations. One of them, appeared to have arisen through the occurrence of a gene conversion event in which the SLA-7 gene served as template. Only MIC-2 gene might be functional, the second MIC-1 gene being truncated. In all, the 14 genes characterized spans 37% of the total sequence. The remaining 63% nucleotides comprised a number of repeat DNA motives, including LINE fragments, SINEs, microsatellites, and also numerous nucleotide stretches not yet defined in swine.