Low-affinity Antigen Research Articles

The CD19/CR2/TAPA-1 complex of B lymphocytes: linking natural to acquired immunity.

B lymphocytes must respond to low concentrations of antigen despite having low affinity antigen receptors during the primary immune response. CD19, a B cell-restricted membrane protein of the immunoglobulin superfamily that associates with the antigen receptor complex, may help the B cell meet this requirement. Cross-linking CD19 to membrane immunoglobulin (mIg) lowers, by two orders of magnitude, the number of mIg that must be ligated to activate phospholipase C (PLC) or to induce DNA synthesis. CD19 is coupled, via protein tyrosine kinases (PTKs), to PLC and phosphatidylinositol 3' kinase (PI3' kinase), and it interacts with the Src-type nonreceptor PTK lyn. It also associates with two other membrane proteins, CR2 (complement receptor type 2, CD21), which permits nonimmunologic ligation of CD19, and TAPA-1, a member of the tetraspan family of membrane proteins. CR2 binds fragments of C3 that are covalently attached to glycoconjugates. This indirectly enables CD19 to be cross-linked to mIg after preimmune recognition of an immunogen by the complement system. CR2 also can be ligated by CD23, a lectin-like membrane protein that resides on cells that may present antigen to B cells. TAPA-1 associates with several other membrane proteins on B and T cells, including MHC class II, CD4, and CD8, and it promotes Ca2(+)- and LFA-1-independent homotypic aggregation when ligated directly or indirectly through CD19 or CR2. This may facilitate interaction of the B cell with other cells essential for cellular activation. The formation of this membrane protein complex by representatives of three different protein families helps the B cell resolve its dilemma of combining broad specificity with high sensitivity.

Immunological function of a defined T-cell population tolerized to low-affinity self antigens

In the thymus there are two major mechanisms of T-lymphocyte tolerance: clonal deletion and clonal inactivation. One important problem underlying the mechanism of clonal inactivation is why unresponsive cells are maintained in the mature peripheral T-cell repertoire. Here we report that transgenic alpha beta T-cells may be tolerized to a self antigen Mls-1a, but still retain proliferative responses for alternative peptide antigens and superantigens. These self-tolerant T cells can also provide immunopathological and memory cytotoxic function in vivo. We propose that high-affinity/avidity self-reactive T cells are deleted in the thymus, whereas lower-affinity/avidity interactions lead to unresponsiveness and define the 'resting threshold' for a given T cell. These low-affinity self-tolerant T cells remain functionally competent for high-affinity foreign antigens, and efficiently eliminate natural pathogens in vivo.

Antibody affinity disturbance in the immunoglobulin immune response to mucosal antigenic stimulation in lung cancer. The Betalactoglobulin Model.

This study is a continuation of a recent study, in which a defect in the immunoglobulin G (IgG) response to some natural antigens (bovine betalactoglobulin [BLG] from cow's milk and antigen p1 from the house dust mite Dermatophagoïdes pteronyssinus), usually presented at the mucosal level, was documented in lung cancer patients. The present study further characterizes this difference in terms of antibody relative functional affinity in the BLG model. Relative functional affinity was evaluated by solid-phase enzyme-linked immunosorbent assay in terms of the relationship between specific IgG retention, assessed with peroxidase-labeled protein A, and serial dilutions of IgG fractions isolated from 24 sera from lung cancer patients and 24 sera from healthy control subjects matched for their anti-BLG IgG antibody titers. The procedure was performed in the presence and absence of low concentrations of diethylamine, which was expected to prevent low-affinity antigen-antibody binding without affecting the binding of high-affinity antibodies. Anti-BLG IgG antibody affinity also was evaluated in 25 patients with early-stage lung cancer, before and after (5 +/- 1 week) complete surgical excision of the tumor. Results, expressed as the slope of the binding curves and their leftward shift induced by diethylamine, showed different antibody populations between the two groups. Control sera showed a heterogeneous population of anti-BLG IgG antibodies, including antibodies of higher (steeper slope) and lower (more gradual slope) functional affinity. Cancer sera exhibited a less heterogeneous population of anti-BLG IgG antibodies, mostly with lower functional affinity. No change was observed in anti-BLG IgG antibody affinity in the 25 lung cancer patients tested 5 +/- 1 week after complete surgical excision of the tumor. These results document a persistent qualitative immunologic disturbance in patients with lung cancer, regardless of the type and extent of tumor. The potential relationship between this observation and the development of lung cancer, however, is presently unknown.

CD19: lowering the threshold for antigen receptor stimulation of B lymphocytes.

Lymphocytes must proliferate and differentiate in response to low concentrations of a vast array of antigens. The requirements of broad specificity and sensitivity conflict because the former is met by low-affinity antigen receptors, which precludes achieving the latter with high-affinity receptors. Coligation of the membrane protein CD19 with the antigen receptor of B lymphocytes decreased the threshold for antigen receptor-dependent stimulation by two orders of magnitude. B lymphocytes proliferated when approximately 100 antigen receptors per cell, 0.03 percent of the total, were coligated with CD19. The B cell resolves its dilemma by having an accessory protein that enables activation when few antigen receptors are occupied.

Physicochemical studies on the antigen-antibody complexes of two monoclonal antibodies against rabbit thymocytes

We have characterized two monoclonal antibodies directed against rabbit thymocyte antigens. Using internally labelled MD, cells (a transformed T-cell line), ART-F immunoprecipitated a membrane glycoprotein of 95,000 mol. wt, while ART-A immunoprecipitated two major glycoproteins, one of 95,000 mol. wt, the other of 105.000mol.wt as analysed on reduced SDS-PAGE. ART-A recognizes 650,000 sites both on rabbit thymocytes and MD 3 cells with an association constant of 1.5 × 10 5/ M. ART-F shows a biphasic binding curve with a high affinity constant of 3.3 and 0.6 × 10 8/ M and a low affinity constant of 2.2 and 6.9 × 10 5/ M for thymocytes and MD 3 cells respectively. The numbers of high affinity and low affinity antigens are 200,000 for the former and 800,000 for the latter. On the basis of the kinetic data, the difference between low and high affinity is attributed to the difference in association-rate constants. The affinity constants calculated from the reaction-rate constants are in good agreement with those obtained from equilibrium measurements. While ART-F prevents the binding of ART-A, ART-A does not inhibit the affinity binding of ART-F. The analysis of the equilibrium, inhibition and kinetic data allow us to present a model for the structural relation of the epitopes recognized on the T-cells by the two monoclonal antibodies.

The immunoregulatory role of cholesterol and other lipids: A hypothesis

Membrane lipids play an important role in cellular responses to exogenous signals. In immunocompetent lymphocytes, marked changes in the concentrations of membrane lipids occur following cell-antigen interaction. These changes lead to an increase in membrane fluidity, thus facilitating the microaggregation of receptor-antigen complexes. This event constitutes the inductive signal for lymphocytes. Lipid profile alterations leading to increased concentration of membrane cholesterol, of polyunsaturated lipids, or of both, bring about a decrease in membrane fluidity. The latter interferes with receptor displacement preventing delivery of an inductive signal to the responding cell. Interference with microaggregation is readily brought about in interactions involving low affinity antigens, such as tumoral antigens. We postulate that in hyperlipidemic and hypercholesterolemic states there is decreased immune responsiveness to weak antigens due to the aformentioned lipid profile alterations in the membranes of immunocompetent cells. The manner in which an increase in the concentration of the lipids mentioned can lead to decreased immune responsiveness and hence to an increased incidence of malignancies in hyperlipidemic and hypercholesterolemic states is the hypothesis presented in this paper.

Immune response to chemically modified flagellin. II. Evidence for a fundamental relationship between humoral and cell-mediated immunity.

Flagellin (mol.wt. 40,000) from S. adelaide organisms and a series of acetoacetyl derivatives of flagellin were tested for their ability to induce humoral and cell-mediated immunity in adult rats. It was found that unmodified flagellin was an excellent inducer of antibody formation but a poor inducer of delayed-type hypersensitivity. In contrast, increasing acetoacetylation steadily destroyed the ability of flagellin to initiate antibody formation but enhanced the capacity of the molecule to induce flagellin-specific cell-mediated immunity and antibody tolerance. In fact, it appeared that in adult rats antibody formation and cell-mediated immunity may well be opposing immunological processes. Furthermore, the affinity of the acetoacetyl flagellins for anti-flagellin antibodies appeared to determine the type of immune response which predominated. High affinity antigen produced antibody formation whereas low affinity antigen induced cell-mediated immunity and antibody tolerance. The importance of affinity was further evidenced by the fact that a CNBr digest of flagellin induced humoral and cellular immune responses identical to an acetoacetylated flagellin of comparable antigenic activity. From these studies it was proposed that both humoral and cell-mediated immunity can be directed against the same antigenic determinants but that the specificity requirements for delayed hypersensitivity (and antibody tolerance) are less than those required for antibody formation. Some remarkable immunological features of the flagellin system were revealed. Flagellin induced comparable delayed-type hypersensitivity when injected in either saline or FCA. Furthermore, FCA only slightly enhanced the delayed responses induced by the acetoacetyl flagellins and in fact these preparations produced antibody tolerance whether injected in saline or adjuvant. Finally, in contrast to the adult tolerance induced by the acetoacetylated flagellins, which existed only at the antibody level, tolerance in neonatal rats existed at the level of both humoral and cell-mediated immunity. This finding is the first indication of a fundamental difference between neonatal and adult tolerance. The significance of these findings is discussed in the light of current immunological concepts and a hypothesis proposed to explain these phenomena.