Specificity and Immunobiological Properties of Monoclonal Antibody IMH2, Established after Immunization with Le^b/Le^a Glycosphingolipid, a Novel Extended Type 1 Chain Antigen¹

Extended lacto-series type 1 chain antigens lacking type 2 chain core have recently been shown to comprise a new type of tumor-associated carbohydrate antigen. Examples are Le^a/Le^a (IV³Galβ1→3[Fucα1→4] GlcNAcLc₄Cer) and Le^b/Le^a (IV³Fucα1→2Galβ1→3[Fucα1→4]Glc-NAcLc₄Cer) (M. R. Stroud, et al., J. Biol. Chem., 266: 8439–8446, 1991; Eur. J. Biochem., 203: 577–586, 1992). We have now established an IgG3 mouse monoclonal antibody (IMH2) after immunization of mice with Le^b/Le^a antigen; however, monoclonal antibody (MAb) IMH2 reacted not only with the immunogen used but also with Le^y/Le^x and to a lesser degree with short-chain Le^y or Le^b with hexasaccharide ceramide (i.e., IV²FucIII³FucnLc₄Cer or IV²FucIII⁴FucLc₄Cer). It showed a high incidence of staining and strong reactivity with carcinomas of colon, rectum, liver, pancreas, and endometrium, but no reactivity with normal colonic mucosa at various loci, and minimal reactivity with normal liver, pancreas, or uterine endometrium. On the other hand, it reacted with normal gastric mucosa, cecal mucosa, urothelium, adrenal glands, and thymus. Its expression in colorectal tumors and normal cecal tissue was independent of secretor status, whereas that in normal urothelium was dependent on secretor status. MAb IMH2 displayed strong lymphocyte-activated or complement-dependent killing of human colonic cancer Colo205 cells in vitro, and inhibition of Colo205 growth in vivo; this inhibition was comparable to that by MAb NCC-ST-421, which is directed to Le^a/Le^a epitope (M. Watanabe, et al., Cancer Res., 51: 2199, 1991). These results indicate that a new extended type 1 chain structure, Le^b/Le^a, is a useful tumor marker associated with carcinomas of colon, rectum, pancreas, liver, and endometrium and that MAb IMH2 has potential diagnostic or therapeutic applicability for these carcinomas.