Antisera to frog immunoglobulins cross-react with a periodate-sensitive cell surface determinant

Abstract

WHILE investigating surface immunoglobulin (Ig) on frog (Rana catesbeiana) lymphocytes by immunofluorescence, we observed, unexpectedly, that several rabbit antisera to frog Ig stained frog thymocytes and all blood cells, including lymphocytes, granulocytes and erythrocytes¹. Absorption with frog erythrocytes abolished the staining of thymocytes and granulocytes, indicating that similar antigenic determinants are present on these cell types. Some peripheral lymphocytes were still stained by the absorbed antiserum, but others were unstained. Evidently, after removal of the cross-reactive antibodies, two lymphocyte populations, possibly homologous to mammalian and avian B and T lymphocytes, can be identified on the basis of surface Ig. Antibodies that react with all blood cells were detected in five of eight antisera to frog high molecular weight (HMW) Ig, but not in six antisera to the anti-genically distinct low molecular weight (LMW) Ig. (Rana catesbeiana have three distinct Ig isotypes: an HMW Ig, which seems similar to mammalian IgM, and two highly cross-reactive LMW Igs, which have an uncertain relationship to mammalian Ig classes^2,3.) The presence of these cross-reactive antibodies in antisera to HMW Ig may lead to the mistaken identification of Ig on cell surfaces. Absorption of such an antiserum with purified frog HMW Ig and with a non-Ig component of frog plasma, but not with frog LMW Ig, also abolished the staining of thymocytes and granulocytes. The presence of similar determinants on the surfaces of several cell types and on at least two plasma components raised the possibility that a carbohydrate structure might be involved. Experiments supporting this possibility are reported here.