Capping and co-capping of membrane immunoglobulin and lipid-conjugated immunoglobulin inserted in the cell membrane of B lymphocytes

B cells carrying surrogate receptors in their membranes process and present antigen to specific murine T cells

A palmitate-conjugate derivative of ovalbumin which can be inserted into the membrane of B cells has been prepared. The ability of these cells to act as antigen-presenting cells for specific T lymphocytes obtained from immunized mice was tested. It was found that the conjugates were more efficiently processed and presented than the naive form of the antigen. Palmitate-conjugated antibodies specific to ovalbumin were also inserted into the cell membrane of normal B lymphocytes. These cells were pulsed with the antigen and tested as antigen-presenting cells for T cells obtained from immunized mice. The antibody-decorated B cells presented ovalbumin more efficiently than non-decorated controls. Whether antibody-decorated, antigen-pulsed B cells could prime T cells in vivo was investigated. Some priming activity was found.

Membrane-incorporated surrogate receptors increase antigen presentation by resting B cells, but not by LPS-activated blasts

Palmitate-conjugated monoclonal antibodies specific to ovalbumin were inserted into the cell membrane of normal resting B cells and LPS-activated blasts. These two decorated B cells were tested for their ability to act as antigen-presenting cells for ovalbumin-specific I-Ad-restricted T-cell hybridomas. It was found that the antibody-decorated resting B cells presented antigen more efficiently than non-decorated controls. However, no increment was observed when decorated LPS blasts were compared with non-decorated blasts. This is explained by the fact that the inserted antibodies quickly disappeared from the cell membrane of LPS blasts, while they were retained for a long period in the membrane of resting B cells.

Membrane-incorporated immunoglobulin receptors increase the antigen-presenting ability of B cells

Monoclonal antibodies specific for ovalbumin were conjugated to palmitate and inserted into the membrane of normal spleen B cells. Their presence in the membrane, as well as their ability to bind ovalbumin, was established by immunofluorescence. The so called anti-ovalbumin-'decorated' B cells were tested for their ability to act as antigen-presenting cells for ovalbumin-specific I-Ad-restricted T-cell hybridomas. It was found that the antibody-decorated B cells presented antigen more efficiently than non-decorated B cells.

Antigen activation of human B lymphocytes bearing artificial antigen receptors

When highly purified human and murine B cells are challenged in vitro with certain so called "T cell-independent" activators such as the polyclonal B cell activator lipopolysaccharide (LPS) or the clonally specific B cell activator dinitrophenyl-conjugated polymerized flagellin (DNP-POL), mouse, but not human, cells differentiate into immunoglobulin-secreting cells. However, results from this study show that DNP-POL can cause human B cell differentiation in a T cell-independent manner when the antigen is concentrated onto the cells via artificially incorporated palmitate-modified anti-DNP mouse IgA molecules. This response is comparable in magnitude to that induced by a T cell-dependent polyclonal B cell activator, pokeweed mitogen, in unfractionated mononuclear cell cultures, suggesting that DNP-POL induced polyclonal B cell differentiation. DNP-POL binding to the artificial receptor molecules on B cells did not cause cellular proliferation, even in unfractionated mononuclear cell populations. These results are similar to those obtained in previous studies using mouse B cells in which the artificial receptor was unable to act as a transmembrane signaling element. From these studies, we conclude that B cells express clonally unrestricted, presumably low-avidity, endogenous receptor for POL, and that signaling through this receptor activates B cell differentiation but not cell proliferation.

The immunoglobulin receptors on B cells bind antigen, focus activation signals to them and initiate antigen-presentation

We do not agree with the analysis of Langman and Cohn on the function of Ig receptors. We have reviewed the available literature regarding anti-Ig activation of B cells and found it contradictory and unconvincing. We have presented experimental evidence on the inability of Ig receptors on B cells to mediate activation or tolerogenic signals. We suggest that the Ig receptors serve to focus antigen to specific B cells so the B cells can be activated by TI antigens or helper T cells. The Ig molecules also bind foreign antigen and thereby initiate internalization and antigen processing. The processed peptides are exported to the membrane, where they associate with MHC class II antigens, thus transforming B cells into efficient antigen-presenting cells.

Thymus-independent B-cell activation: passively incorporated membrane antibodies serve to focus the antigen but cannot transmit activation signals

We have activated B cells with membrane-incorporated monoclonal antibodies against the hapten trinitrophenyl (TNP) using various concentrations of the polyclonal activator lipopolysaccharide (LPS) haptenated with TNP. We found that anti-TNP-decorated B cells were polyclonally activated by 1000-fold lower concentrations of TNP-LPS than untreated B cells or B cells decorated with antibodies of other specificities. In order to test whether the passively incorporated anti-TNP monoclonals could mediate activation signals or only served to focus the polyclonal activator TNP-LPS to the B cells, we compared the response of anti-TNP-decorated B cells from normal C3H/He mice and from the LPS-unresponsive strain C3H/HeJ. We found that B cells from C3H/HeJ mice could not be activated to polyclonal antibody synthesis, in contrast to B cells from the LPS-responsive strain C3H/He. Thus, contrary to what was previously suggested, the incorporated anti-TNP antibodies could not mediate activation signals, but only served to passively bind and concentrate TNP-LPS to the membrane of the B cells, thereby increasing the concentration of the polyclonal B-cell activator LPS to the B cells.