The glycans on enveloped viruses are synthesized by host-cell machinery. Some of these glycans on zoonotic viruses of mammalian reservoirs are recognized by human natural antibodies that may protect against such viruses. These antibodies are produced mostly against carbohydrate antigens on gastrointestinal bacteria and fortuitously, they bind to carbohydrate antigens synthesized in other mammals, neutralize and destroy viruses presenting these antigens. Two such antibodies are: anti-Gal binding to -gal epitopes synthesized in non-primate mammals, lemurs, and New World monkeys, and anti-N-glycolyl neuraminic acid (anti-Neu5Gc) binding to N-glycolyl-neuraminic acid (Neu5Gc) synthesized in apes, Old World monkeys, and many non-primate mammals. Anti-Gal appeared in Old World primates following accidental inactivation of the 1,3galactosyltransferase gene 20-30 million years ago. Anti-Neu5Gc appeared in hominins following the inactivation of the cytidine-monophosphate-N-acetyl-neuraminic acid hydroxylase gene, which led to the loss of Neu5Gc <6 million-years-ago. It is suggested that an epidemic of a lethal virus eliminated ancestral Old World-primates synthesizing -gal epitopes, whereas few mutated offspring lacking -gal epitopes and producing anti-Gal survived because anti-Gal destroyed viruses presenting -gal epitopes, following replication in parental populations. Similarly, anti-Neu5Gc protected few mutated hominins lacking Neu5Gc in lethal virus epidemics that eliminated parental hominins synthesizing Neu5Gc. Since -gal epitopes are presented on many zoonotic viruses it is suggested that vaccines elevating anti-Gal titers may be of protective significance in areas endemic for such zoonotic viruses. This protection would be during the non-primate mammal to human virus transmission, but not in subsequent human to human transmission where the virus presents human glycans. In addition, production of viral vaccines presenting multiple -gal epitopes increases their immunogenicity because of effective anti-Gal-mediated targeting of vaccines to antigen presenting cells for extensive uptake of the vaccine by these cells.
Human natural antibodies to mammalian carbohydrate antigens (MCA) bind to carbohydrate-antigens synthesized in other mammalian species and protect against zoonotic virus infections. Three such anti-MCA antibodies are: (1) anti-Gal, also produced in Old-World monkeys and apes, binds to -gal epitopes synthesized in non-primate mammals, lemurs, and New-World monkeys; (2) anti-Neu5Gc binds to Neu5Gc (-glycolyl-neuraminic acid) synthesized in apes, Old-World monkeys, and many non-primate mammals; and (3) anti-Forssman binds to Forssman-antigen synthesized in various mammals. Anti-viral protection by anti-MCA antibodies is feasible because carbohydrate chains of virus envelopes are synthesized by host glycosylation machinery and thus are similar to those of their mammalian hosts. Analysis of MCA glycosyltransferase genes suggests that anti-Gal appeared in ancestral Old-World primates following catastrophic selection processes in which parental populations synthesizing -gal epitopes were eliminated in enveloped virus epidemics. However, few mutated offspring in which the 1,3galactosyltransferase gene was accidentally inactivated produced natural anti-Gal that destroyed viruses presenting -gal epitopes, thereby preventing extinction of mutated offspring. Similarly, few mutated hominin offspring that ceased to synthesize Neu5Gc produced anti-Neu5Gc, which destroyed viruses presenting Neu5Gc synthesized in parental hominin populations. A present-day example for few humans having mutations that prevent synthesis of a common carbohydrate antigen (produced in >99.99% of humans) is blood-group Bombay individuals with mutations inactivating H-transferase; thus, they cannot synthesize blood-group O (H-antigen) but produce anti-H antibody. Anti-MCA antibodies prevented past extinctions mediated by enveloped virus epidemics, presently protect against zoonotic-viruses, and may protect in future epidemics. Travelers to regions with endemic zoonotic viruses may benefit from vaccinations elevating protective anti-MCA antibody titers.
Tolerance induction to transplantation-associated carbohydrate antigens, such as blood group A or B and the alpha-gal epitope (Gal(alpha)1-3Gal(beta)1-4GlcNAc-R), is of clinical significance. This study demonstrates tolerance induction to the alpha-gal epitope in the experimental animal model of alpha1,3galactosyltransferanse knockout mice (KO mice) lacking alpha-gal epitopes by administering syngeneic lymphocytes expressing alpha-gal epitopes. Repeated immunization of control KO mice with pig kidney membranes (PKM) expressing many alpha-gal epitopes induces an extensive anti-Gal antibody response against this epitope. In contrast, KO mice that received as few as 2 x 10(6) wild-type (WT) lymphocytes were tolerized and failed to produce anti-Gal following PKM immunizations. Accordingly, control mice producing anti-Gal rapidly rejected transplanted WT hearts, whereas tolerized mice did not reject WT hearts. These findings suggest that autologous blood lymphocytes processed to express a carbohydrate antigen may induce a similar tolerance to such an antigen upon administration into humans.
Alpha1,3galactosyltransferase (alpha1,3GT) synthesizes alpha-gal epitopes (Gal(alpha)1-3Galbeta1-4GlcNAc-R) on glycoconjugates in nonprimate mammals but not in humans. Transduction of alpha1,3GT gene into human HeLa cells by an adenovirus vector allowed for accurate kinetics studies on the appearance of alpha1,3GT and of its product, the alpha-gal epitope, in the transduced cells. Mouse alpha1,3GT cDNA was inserted into a replication-defective adenovirus vector. This viral vector, designated Ad(alpha)GT, could be propagated in human 293 cells that have the viral E1 complementing gene. Transduction of HeLa cells resulted in immediate penetration of approximately 20 Ad(alpha)GT copies into each cell and the appearance of alpha1,3GT mRNA after 4h. Catalytic activity of alpha1,3GT was first detected in the cells after 6 h. The initial appearance of alpha-gal epitopes (approximately 6 x 10(4)/cell) on cell surface glycoconjugates was detected 10 h posttransduction, whereas 24 h posttransduction each cell expressed 2 x 10(6) epitopes. The activity of alpha1,3GT in cells transduced with approximately two copies of Ad(alpha)GT was eightfold lower than that in cells transduced with approximately 20 Ad(alpha)GT copies; however, the number of alpha-gal epitopes/cell remained closely similar. This implies that increased alpha1,3GT activity above a certain saturation level does not result in a corresponding increase in the carbohydrate product, possibly because of competing glycosyltransferases.
The most prevalent anticarbohydrate antibodies in human serum are anti-Gal interacting specifically with the alpha-gal epitope (Galalpha1-3Galbeta1-4GlcNAc-R) and anti-blood group antibodies interacting with blood group A and B antigens. The alpha-gal epitope, although absent in humans, comprises part of the core of carbohydrate chain in A and B antigens. Therefore, it was of interest to determine whether immunoglobulin (Ig) G antibodies, elicited in patients rejecting ABO-incompatible kidney allografts, can interact with the alpha-gal epitope. Anti-A and anti-B antibodies were determined by enzyme-linked immunosorbent assay (ELISA) with blood group A or B human red cell membranes, as solid phase antigens. Anti-Gal was determined by ELISA with alpha-gal-bovine serum albumin as solid-phase antigen. Specific removal of anti-Gal was performed by adsorption on fixed rabbit red cells. Blood group O patients who underwent transplantation with either A or B kidney produced an antibody that bound to all three carbohydrate antigens. This multispecific antibody, designated anti-Gal A/B, is specific to the core alpha-gal epitope within A and B antigens. Recipients of allograft expressing incompatible blood group B also produce anti-Gal B antibody, which binds to the core alpha-gal epitope only in the B antigen. Anti-Gal A/B and anti-Gal B constitute most of the elicited anti-blood group antibody response. Allograft recipients also produced pure anti-A, or pure anti-B, which require the complete blood group structure for binding. The findings in this study imply that much of the immune response elicited by incompatible A or B antigens on kidney allografts results in activation of anti-Gal B-cell clones producing antibodies to the core alpha-gal epitope in these blood group antigens. Only less than 25% of the elicited antibodies interact with the complete A or B antigens (i.e., pure anti-A or pure anti-B). These findings suggest that prevention of the anti-Gal response may decrease the immune rejection of ABO-incompatible allografts.