Antigen presentation is enhanced by targeting antigen to the Fc epsilon RII by antigen-anti-Fc epsilon RII conjugates.

Targeting Ag to the Fc epsilon RII by Ag-specific IgE has been shown to be an efficient means of enhancing Ag presentation by B cells to Ag-specific T cells. To take advantage of the Fc epsilon RII as a targeting molecule and to investigate whether IgE was required for mediation of the enhanced stimulation, Ag was covalently coupled to anti-Fc epsilon RII by using heterobifunctional crosslinking reagents. These Ag-Ab conjugates were used with T cell lines specific for the Ags, OVA (BB6.5) or rabbit gamma-globulin (CDC35 and D1.6), and splenic B cells to examine both B cell and T cell proliferation in vitro. Significant presentation of Ag-anti-Fc epsilon RII conjugates was apparent at doses of Ag 1,000- to 10,000-fold lower than seen with unconjugated Ag alone. Ag presentation with the use of anti-Fc epsilon RII-Ag conjugates was as good as or better than conjugates with Ab to the adhesion molecule Pgp-1 or control Ab in T cell proliferation and better than those conjugates in B cell proliferation assays (10- to 100-fold). Anti-Fc epsilon RII-Ag conjugates were clearly more effectively presented than Ag-anti-Fc gamma RII conjugates (> 100-fold). Mouse Fc epsilon RII is presently known to be expressed on B cells and follicular dendritic cells and these in vitro results suggest that the conjugates would be useful tools for investigating the role of IgE-mediated B cell Ag presentation in vivo. BALB/c mice immunized with OVA-anti-Fc epsilon RII conjugates made a quite significant OVA-specific IgG1 response and a detectable IgE response. No detectable Ab was produced in response to OVA alone and a minimal response was seen when an isotype-matched control conjugate was used. Thus, the results indicate that Fc epsilon RII targeting is operative both in vivo and in vitro.

Chromosomal location and isoform analysis of mouse Fc epsilon RII/CD23.

The gene for the mouse low affinity receptor for IgE (Fc epsilon RII, also known as CD23) was mapped on Chromosome (Chr) 8 proximal to Plat. This gene, symbolized Fcer2 (formerly Fce2) resides in a region of Chr 8 with linkage homology with human chromosomes 8 and 19. The mouse Fc epsilon RII was examined for the presence of alternate N-terminal forms such as seen in humans. An antisense RNA probe was prepared from the 5' end of the cDNA through the first 660 bp of the cDNA and was used to analyze message from Fc epsilon RII+ B cells and B cell hybridomas both before and after treatment with interleukin 4 (IL-4). Using RNase protection analysis, a major 640 bp band corresponding to the full length probe was seen, even after activation of the cells with LPS in the presence of IL-4, which is known to give high expression levels of the Fc epsilon RII. This result suggests that the mouse does not produce significant levels of an alternate IL-4 inducible Fc epsilon RII, as seen in man, and this may explain the more restricted cell lineage expression of the Fc epsilon RII in the mouse.

Fc epsilon receptors.

Advances in our understanding of the molecular structure of Fc receptors have been made at a rapid pace. Details of how Fc receptors are involved in cell triggering, e.g. allergic mediator release from mast cells, and IgE synthesis are also continuing to be elucidated, although much work is still required. Recent highlights of investigations of mast-cell and lymphocyte IgE Fc receptors will be outlined.