Membrane immunoglobulins of vertebrate lymphocytes

Immunoglobulin-bearing blood leucocytes in the Pacific hagfish

The occurrence of immunoglobulin (Ig) bearing leucocytes in the blood of the Pacific hagfish, Eptatretus stoutii , was examined using a murine monoclonal antibody (45.3) and a rabbit antiserum specific for hagfish serum Ig. Binding of antibody 45.3 to hagfish leucocytes assessed by radioimmunoassay was inhibited by preincubation of antibody with purified serum Ig thus verifying the presence of cell surface Ig cross reactive with serum Ig. The monoclonal antibody identified approximately 65% of blood leucocytes as Ig+ve while the rabbit antiserum indicated 81% Ig+ve cells. Both antibody preparations failed to react specifically with cells from mouse, horned shark, tunicate or sea star; this indicates the distinctive nature of hagfish Ig. The high percentage of blood cells bearing surface Ig in the hagfish raises the possibility that lymphocyte divergence to separate B and T pathways may not have occurred in this most primitive vertebrate. Alternatively, an Ig-like specificity characteristic of both "T" and "B" lymphocytes may have been detected. In any event, a subset of Ig negative leucocytes is evident in hagfish.

Molecular analysis of the lymphocyte membrane

Methods for the molecular analysis of lymphocyte membranes are reviewed briefly, and wherever possible presented in a manner relevant to comparative studies. The specific areas reviewed include the bulk isolation of lymphocyte membranes, the use of radioisotopes to covalently label lymphocyte membrane molecules, the use of lectins to characterize membrane glycoconjugates, and our current understanding of lymphocyte membrane immunoglobulins.

Lymphocyte heterogeneity in the trout, Salmo gairdneri, defined with monoclonal antibodies to IgM

A monoclonal antibody to trout serum IgM was tested by immunofluorescence analysis with lymphocytes from thymus, spleen and head kidney. By visual examination, the antibody reacted with only a subpopulation of lymphocytes. The mean values +/- SE for positive cells were 5.2 +/- 2.3% in the thymus, 30.3 +/- 7.9% in the spleen and 12.4 +/- 3.0% in the head kidney. Flow cytofluorometric analysis revealed evidence of heterogeneity by size among the membrane IgM-positive cells of the head kidney and spleen. Depletion of head kidney cells positive for surface IgM by an immune affinity adherence technique of panning, using monoclonal anti-IgM, significantly reduced the mitogenic response to lipopolysaccharide but not to concanavalin A. It is suggested that this information supports the existence of distinct subpopulations of fish lymphocytes that may be homologous in certain respects to mammalian T and B type cells.

Spatial structure of immunoglobulin molecules

Immunoglobulin molecules of the class G (antibody molecules) consist of two heavy chains (50,000 dalton molecular weight) and two light chains (25,000 dalton). The overall shape is a Y with the arms formed by the light chains and the N-terminal half of the heavy chains in tight association. The stem is formed by the C-terminal halves of the heavy chains. The heavy and the light chains fold into globular domains of molecular weights of 12,000 dalton. There are four domains of the heavy chain and two of the light chain. All these domains show a similar fold, consisting of two B-sheets but display considerable differences in detail. The N-terminal variable domains of heavy and light chains and specifically the hypervariable polypeptide segments of the domains, located at the tips of the Y, constitute the antigen and hapten binding site. The nature of the amino acid residues of the hypervariable loops determines the shape and the specificity of the antibody. All domains pair tightly laterally, except the CH2 domains of the heavy chain. This domain has carbohydrate bound which prevents lateral association. Longitudinal interaction between the domains is loose and allows flexibility in the arrangement. Flexibility is probably of significance for antibody function. Arm (Fab) and stem (Fc) parts are linked by the hinge peptide which contains a segment with a unique conformation of two parallel poly-proline helices. Antigen binding triggers effector functions of antibodies. Antigen binding is at the tips of the Y-shaped antibody, but effector functions are displayed by the stem part. It is an open question whether conformational changes of the antibody molecule play a significant role in the trigger mechanism.