University of Massachusetts Medical School
Anti-Gal and anti-non Gal antibodies to ECM implants
1UMass Medical School, Worcester, MA, United States
Tissue engineering by the use of porcine extracellular matrix (ECM) implants, as well as ECM from other nonprimate mammalian origin, may be compromised by the same antibody barriers as those in xenotransplantation: anti-Gal and anti-non gal antibodies. The alpha-gal epitope, which is the ligand of anti-Gal, is abundant in nonprimate mammals ECM glycoproteins and proteoglycans, in particular in ECM comprised of basement membranes. Moreover, anti-Gal antibody titers in human recipients of mammalian implants expressing alpha-gal epitopes, increase within 2-4 weeks post implantation by 10-100 folds. In addition, most nonprimate mammalian proteins (including most ECM proteins and proteoglycans) are immunogenic in humans since they contain varying proportions of amino acid sequences that differ from those in human homologous proteins. Therefore, ECM implants also elicit production of anti-non gal antibodies in human recipients. Formation of anti-non gal antibodies is significantly slower than that of anti-Gal antibodies since the initial size of each of the anti-non gal B cell clones is much smaller than that of anti-Gal B cell clones (~1% of B cells). Both anti-Gal and anti-non gal antibodies may be detrimental to implant regeneration since their binding to ECM activates the complement system and thus induces recruitment of macrophages. These macrophages bind to the immunocomplexed antibodies and mediate degradation of the implant. Moreover, these antibodies can hinder interaction of stem cells with the ECM, thereby preventing stem cells from receiving the appropriate cues from ECM molecules for their differentiation into cells restoring the target tissue. Such anti-Gal and anti-non gal antibody hindrance of stem cell/ECM interaction may ultimately result in fibrosis instead of regeneration of structure and function of the tissue. Anti-Gal detrimental effects can be avoided by the use of ECM implants lacking alpha-gal epitopes, either by destruction of these epitopes with recombinant alpha-galactosidase, or by using ECM from alpha1,3galactosyltransferase knockout pigs. Anti-Gal may further be harnessed for minimizing or avoiding the detrimental effects of anti-non antibodies on ECM implants. This may be achieved by accelerating recruitment of stem cells with biodegradable alpha-gal nanoparticles administered within the ECM implant. These nanoparticles present multiple alpha-gal epitopes. Following implantation of ECM containing these nanoparticles, anti-Gal binds to the multiple alpha-gal epitopes on the nanoparticles and activates the complement system. This activation generates complement cleavage chemotactic peptides that induce rapid recruitment of macrophages. The recruited macrophages are activated as a result of the interaction between their Fc receptors and Fc portion of anti-Gal coating the alpha-gal nanoparticles. These activated macrophages produce pro-healing cytokines/growth factors that recruit stem cells to the ECM implant. The recruited stem cells are instructed by the ECM molecules to differentiate into cells that regenerate the tissue. Because of this rapid recruitment of stem cells by alpha-gal nanoparticles, the regeneration of the tissue from the implanted ECM can occur prior to the production of anti-non gal antibodies at detrimental titers that can destroy the ECM implant and hinder its interaction with stem cells.
Reference: Galili, U. Avoiding Detrimental Human Immune Response Against Mammalian Extracellular Matrix Implants. Tissue Engineering, Part B: Reviews, 21: 231-241, 215.