T cells in mice expressing a transgenic human TCR beta chain get positively selected but cannot be activated in the periphery by signaling through TCR.

TCR-CD3 complex-mediated signaling is crucial for both developmental selection and antigenic activation of T cells. We report that mice expressing a recombined human TCRbeta chain (Tg), which have normal development of T cells, mounted very weak responses to immunization with protein antigens as well as the HA307-319 peptide recognized by the human T cell clone HA1.7 from which the transgene is derived. An anti-CD3epsilon mAb triggered equivalent proliferation from Tg and non-Tg T cells, but an anti-human TCRbeta mAb induced proliferation poorly in Tg T cells in contrast to human T cells or HA1.7. In Tg mice, T cells expressing endogenous TCR were CD44(high), whereas most transgene-expressing T cells remained CD44(low), suggesting that transgene-expressing cells are not activated in the periphery to participate in immune responses. However, anti-human TCRbeta could induce some activation markers on T cells and cross-linking of the Tg TCR by plate-coated anti-human TCRbeta efficiently induced T cell proliferation. Human TCRbeta-mediated Tg T cell activation could be rescued by exogenous IL-2, as well as by the calcium ionophore A23187, but not by phorbol esters. Thus, this human TCRbeta chain functions efficiently for positive selection of mouse T cells, but not for their peripheral activation, probably because of a lack of oligomerization leading to defects in signaling for calcium flux and IL-2 induction. The data thus suggest an early point of separation of signaling pathways between positive selection and peripheral activation of T cells.

Expression of an immunoglobulin heavy chain transgene in macrophage as well as lymphocyte lineages in vivo.

A rearranged immunoglobulin heavy chain (IgH) transgene-encoded protein is expressed in macrophage lineage cells, in addition to B and T lineages, in transgenic mouse bone marrow. Peripheral macrophages also express transgenic IgH protein. Mature T cells express lower levels than immature thymocytes. Almost all B220+ cells in the bone marrow express transgenic IgH protein, and this early expression in the B lineage is accompanied by a reduction of cell frequency even in the early B220+ CD43+ BP-1- stages, although it is more prominent in BP-1+ pre-B cells. Thus, an IgH transgene can be expressed not only in lymphoid but also in myeloid cells, although its developmental effects are restricted to the B cell lineage.

Presence of activated antigen-binding B cells during immunization enhances relative levels of IFN-gamma in T cell responses.

To examine the influence of Ag presentation by B cells on immune responses, we have used mice transgenic for an Ig heavy chain from a monoclonal anti-azobenzenearsonate (Ars) Ab to deliver Ag to B cells during immunization. A large proportion of transgene-expressing B cells in these mice binds Ars, while transgenic serum Ig shows poor Ars binding. Transgenic B cells present Ars proteins better than their nonhaptenated counterparts. This is associated with an increase in the proliferative responses of transgenic T cells to Ars protein immunization. Although B cell numbers in the transgenic mice are lower, many B cells in them show an activated phenotype, as identified by altered surface levels of peanut agglutinin reactivity, CD23, CD24, CD44, CD62L, and CD86. Even against nonhaptenated immunogens, transgenic responses show significant enhancement in the relative proportions of the Th1 cytokine IFN-gamma over the Th2 cytokines IL-4 and IL-10. Haptenated immunogens further enhance the predilection of transgenic mice to produce relatively more IFN-gamma. Consistent with this, there is an increase in IgG2a/IgG1 ratios in serum Abs in response to haptenated immunogens in transgenic mice. Adoptive transfer of primed hapten-specific secondary B cells into nontransgenic mice also induces an increase in relative levels of IFN-gamma in response to haptenated immunogens. Thus, presentation of immunogen in vivo by activated Ag-binding B cells contributes to enhanced immunogenicity and a Th1 cytokine bias.