The role of antigen-presenting B cells in T cell priming in vivo. Studies of B cell-deficient mice.

Mice rendered B cell deficient by treatment with rabbit anti-mouse IgM (anti-mu) antibodies from birth fail to respond when primed with soluble protein antigens in CFA, as measured by T cell proliferation when challenged with antigen in vitro. The role of B cells in T cell priming in vivo was examined by adoptively transferring hapten-specific B cells into anti-mu mice, followed by immunization with haptenated Ag in CFA. The T cell proliferative response to OVA of anti-mu BALB/c mice was partially restored by the administration of TNP or FITC-specific B cells and immunization with TNP-OVA or FITC-OVA, respectively. This reconstitution was Ag-specific, inasmuch as hapten-binding B cells restored the T cell responses to OVA in mice immunized with the same hapten coupled to OVA. The mechanism of B cell reconstitution of T cell priming in anti-mu mice was addressed using parental to F1 B cell transfers. The Ia restriction pattern of the activated T cells from these mice indicated that both direct presentation of Ag by transferred B cells and antibody-mediated enhancement of Ag presentation by non-B, host Ag-presenting cells occurred. Thus, Ag-specific B lymphocytes play a critical role in priming of T cells in vivo.

Heterogeneity of helper/inducer T lymphocytes. II. Effects of interleukin 4- and interleukin 2-producing T cell clones on resting B lymphocytes.

To compare the helper function of murine T cell clones that secrete IL-2 and IFN-gamma (Th1 cells) or IL-4 and IL-5 (Th2), purified resting B cells were stimulated with F(ab')2 rabbit anti-mouse Ig (RAMG) and rabbit Ig-specific, class II MHC-restricted cloned T cells belonging to the two subsets. Both Th2 clones examined induced strong proliferative responses of B cells in the presence of RAMG, as well as the secretion of IgM and IgG1 antibodies. In contrast, the Th1 clones tested failed to stimulate B cell growth or antibody secretion. Th2-mediated B cell activation was dependent on IL-4 and IL-5, and was also inhibited by IFN-gamma or IFN-gamma produced by Th1 cells present in the same cultures. However, the failure of Th1 cells to help resting B cells could not be reversed with neutralizing anti-IFN-gamma antibody. In addition to this inhibitory effect, IFN-gamma was required for the secretion of IgG2a antibody, particularly when B cells were stimulated with polyclonal activators such as LPS. Finally, both sets of T cell clones secreted lymphokines when stimulated with purified B cells and RAMG. These experiments demonstrate that T cells that differ in lymphokine production also differ in their ability to help B cells as a result of cognate interactions at low concentrations of antigens. Moreover, IL-4, IL-5, and IFN-gamma serve different roles in the T cell-dependent proliferative and differentiative responses of resting B lymphocytes.

Antigen presentation by hapten-specific B lymphocytes. V. Requirements for activation of antigen-presenting B cells.

Splenic B cells specific for the haptens, 2,4,6-trinitrophenyl (TNP) or fluorescein isothiocyanate (FITC) were cultured with a range of concentrations of unmodified or TNP- or FITC-conjugated conalbumin and the conalbumin + I-Ak-specific, interleukin (IL) 1-dependent helper T cell clone, D10 . G4, in the presence and absence of IL-1. Lymphokine secretion, T cell proliferation, and antibody secretion by B cells all exhibited identical antigen dose responses. Thus, hapten-binding B cells presented low concentrations of haptenated conalbumin for activation of both the T and the antigen-presenting B cells. Whereas proliferation of D10 . G4 required the addition of IL-1, both lymphokine production and stimulation of B cells to antibody secretion occurred without exogenous IL-1. These results demonstrate that when B lymphocytes function as presenting cells for antigens that bind to their immunoglobulin receptors, activation of the responding T cells and the B cells themselves occur at similar concentrations of antigen. Moreover, for functional T-B interactions, antigen-presenting B and responding T lymphocytes constitute a complete system that requires no other accessory stimuli, whereas clonal expansion of T cells is dependent on accessory factors such as IL-1. Finally, since D10 . G4 secretes IL-4 but neither IL-2 nor interferon-gamma, our results demonstrate that differentiation of B cells as a consequence of direct ("cognate") interactions with helper T cells as well as of bystander B cells can occur in the absence of IL-1, IL-2, and interferon-gamma.

Antigen presentation by hapten-specific B lymphocytes. III. Analysis of the immunoglobulin-dependent pathway of antigen presentation to interleukin 1-dependent T lymphocytes.

The ability of antigen-specific murine B lymphocytes to present antigen to a normal (nontransformed) antigen-specific, interleukin (IL)1-dependent T cell clone and to heterogeneous T lymphocytes was investigated. Hapten-specific lymphocytes present protein antigens to both the D10G.4.1 T cell clone and heterogeneous immune T cells. When the antigen bears an epitope recognized by the specific B cell, the subsequent presentation of this antigen is 10(2)-10(4)-fold more efficient, as compared to the same, but nonhaptenated protein. The requisite B lymphocyte binds hapten, is radiosensitive and is nonadherent to plastic. The hapten-specific antigen presentation is blocked by antibodies to the surface Ig receptor, while the presentation of unmodified protein is unaffected. These observations are identical to our findings in studies of B cell antigen presentation to T-T hybridomas and therefore demonstrate the generality of the immunoglobulin-dependent pathway of presentation. However, in contrast to the results with T-T hybridomas, the specific B lymphocytes are necessary, but not sufficient, to activate IL 1-dependent T cells. A third cell is required for both B lymphocyte immunoglobulin-dependent and nonspecific antigen-presentation. This cell is radioresistant, plastic adherent and of low density. The requirement for this third cell can be circumvented by supplementing cultures with highly purified IL 1. These results demonstrate that the remarkably efficient ability of B lymphocytes to present antigen is operative with factor-dependent T lymphocytes. Furthermore, conventional accessory cells also appear to play a role in this process, which is consistant with their known requirement in antigen-specific T-B interactions in the generation of antibody responses.