Immunoglobulin variable region sequences of two human monoclonal antibodies directed to an onco-developmental carbohydrate antigen, lactotetraosylceramide (LcOse₄ Cer)

A human monoclonal antibody, 11-50, was generated and was shown to recognize an onco-developmental carbohydrate antigen, LcOse₄ Cer. The isotype of this antibody was IgM, λ, similar to the previously known human anti-LcOse₄ antibodies, such as IgM^WOO and HMST-1. We raised a murine anti-idiotypic antibody G3 (IgG₁, κ) against 11-50, and tested its reactivity towards the affinity purified human polyclonal anti-LcOse₄ antibodies prepared from pooled human sera using a Galβ1 → 3GlcNAcβ-immobilized column. The results indicated that at least a part of the human polyclonal anti-LcOse₄ antibodies shared the G3 idiotype with 11-50. We further analyzed the sequence of variable regions of the two anti-LcOse₄ antibodies, 11-50 and HMST-1. Sequence analysis of the heavy chain variable regions indicated that the V_H regions of these two antibodies were highly homologous to each other (93.5% at the nucleic acid level), and these antibodies utilized the germline genes V_H1.9III and hv 3005f3 as the V_H segments, which are closely related germline genes of the V_HIII family. It was noted that these germline V_H genes are frequently utilized in fetal B cells. The J_H region of both antibodies was encoded by the J_H4 gene. For the light chain, the Vλ segments of the two antibodies were 96.3% homologous to each other at the nucleic acid level. The Vλ segments of both antibodies showed the highest homology to the rearranged Vλ gene called VλII.DS among reported Vλ genes, while the exact germline Vλ genes encoding the two antibodies were not yet registered in available sequence databanks. The amino acid sequences of the Jλ segments of both antibodies were identical. These results indicate that the two human antibodies recognizing the onco-developmental carbohydrate antigen Lc₄ are encoded by the same or very homologous germline genes.