Mechanisms of T-cell-induced allotypic suppression of mouse IgG2a(b) and of tolerance acquisition to this allotype

Role of CD40 in a T cell-mediated negative regulation of Ig production

To investigate the possible role of CD40 in a negative regulation of Ig production, we used the mouse Ig allotype suppression model. T splenocytes from IGH(a/a) mice are able in vivo to totally and chronically inhibit the production of IgG(2a)(b) (IgG2a from the IGH(b) haplotype). Accordingly, postnatal transfer of IGH(a/a) T splenocytes into histocompatible IGH(a/b) F(1) or congenic IGH(b/b) mice leads to a characteristic IgG(2a)(b) suppression. The helper action of anti-IgG(2a)(b) CD4(+) T cells is required for the recruitment of anti-IgG(2a)(b) CD8(+) T suppression effectors. The latter use perforin (pore-forming protein, Pfp)- and/or Fas-dependent cytotoxic pathways to continuously eliminate B cells recently committed to IgG(2a)(b) production. In the present study we first showed that in vivo agonistic anti-CD40 mAb treatment of IGH(a/a) mice, deprived of their CD4(+) T cell compartment, could bypass the help of Ig allotype-specific CD4(+) T cells and generate CD8(+) T effector cells able to strongly inhibit IgG(2a)(b) production. This result demonstrates the usefulness of CD40 triggering in setting up an immune regulatory mechanism. Furthermore, with regard to the suppression-effector mechanism, we demonstrated that B cell CD40 expression was required for full suppression establishment via the Fas-dependent pathway. Indeed, IGH(a/a) PFP(degrees/degrees) T cells (using exclusively the Fas pathway) induced full IgG(2a)(b) suppression against IGH(b/b) CD40(+/+) B cells, but only partial inhibition of IgG(2a)(b) production against IGH(b/b) CD40(degrees/degrees) B cells. This finding provides the first demonstration of direct involvement of B cell CD40 expression in in vivo negative control of an Ig production.

Autoreactive isotype-specific T cells determine B cell frequency

Suppressive activities involving T-B and T-T cell interactions are important to maintain immune system homeostasis. Negative control of IgG2ab+ B cells by anti-IgG2ab T cells derived from Igha mice has been well documented. Nevertheless the real contribution of anti-IgG2ab T cells, endogenously matured in Ighb mice, in controlling IgG2ab+ B cell function has never been investigated. We previously generated anti-IgG2ab TCR-transgenic mice and showed that transgenic T cells were not deleted in the thymus and that they were responsible for a complete and chronic IgG2ab suppression. Here we show that T cells expressing high density of anti-IgG2ab TCR were positively selected in the thymus with a higher efficiency in animals expressing IgG2ab, reached peripheral lymphoid organs and negatively controlled IgG2ab serum levels. Moreover, anti-IgG2ab T cells transgenic for the single TCR chain, thus undergoing normal rearrangements and normal processes of selection, also reached the periphery and suppressed IgG2ab. Interestingly, concentration of IgG2ab in serum inversely correlated with the peripheral frequency of Ig-specific T cells. Finally, T cells able to suppress IgG2ab were obtained from Ighb non-transgenic mice, indicating that anti-2ab T cells are naturally present in the periphery of Ighb animals. We propose that IgG2ab-specific T cells contribute to determine IgG2ab serum levels in Ighb mice.

Non-overlapping Fas- and BCL-2-regulated death pathways in IgG2a(b)-producing B cells

Using perforin (Pfp)- and/or Fas-dependent cytotoxic pathways, T splenocytes from Igh(a/a) mice are able in vivo to totally and chronically eliminate congenic Igh(b/b) B cells committed to IgG2a(b) production. This phenomenon leads to a characteristic absence of serum IgG2a(b) expression (IgG2a(b) allotype suppression) in, for instance, histocompatible Igh(a/b) or Igh(b/b) mice, having neonatally received such T cells. Because the study of the protective role of BCL-2 oncoprotein against Fas-mediated cell death has generated contradictory findings, we examined the possible impact of constitutive overexpression of transgenic human BCL-2 protein in Igh(b/b) B cells when the latter were exposed in vivo exclusively with the Fas-dependent, anti-IgG2a(b) T cell activity of Igh(a/a) Pfp(0/0) mice. We observed that, despite high intracellular expression of functional transgenic BCL-2 and no up-regulation of the principal BCL-2 inhibitors in whole Igh(b/b) B cells, total, chronic and specific IgG2a(b) suppression was exerted by Igh(a/a) Pfp(0/0) cytotoxic T cells. These data show that, in this model of negative regulation of Ig production, Fas- and BCL-2-regulated mechanisms belong to non-overlapping death pathways at the level of IgG2a(b)-producing B cells, targets of Igh(a/a) T cell-mediated cytotoxicity. Thus, in these mature B cells, the Fas signaling-directly operating via caspase 8-does not involve a mitochondria-dependent pathway regulated by BCL-2.

Evidence of Alternative or Concomitant Use of Perforin- and Fas-Dependent Pathways in a T Cell-Mediated Negative Regulation of Ig Production

To study the possible involvement of perforin (Pfp)- and/or Fas-dependent cytotoxicity pathways in a T cell-mediated negative regulation of Ig production, we used the T cell-induced Ig-allotype suppression model. T splenocytes from Igha/a mice, when neonatally transferred into histocompatible Igha/b F1 or Ighb/b congenic hosts, are intrinsically able to totally, specifically, and chronically suppress the production of IgG2a of the Ighb haplotype (IgG2ab). It has not been established whether the suppression effectors, which are anti-IgG2ab MHC class I-restricted CD8+ T cells, cytolyse IgG2ab+ B targets or whether they only silence Ig production. In this study, using T cells from Igha/a Pfp+/+ or Pfpo/o mice, the latter obtained by crossbreeding, and B cells from Ighb/b Fas+/+ or Faslpr/lpr (lymphoproliferation) mice in appropriate adoptive transfer models, we demonstrated that: 1) under blockage of the Pfp-mediated pathway, Igha/a T cells were still able to induce suppression against wild-type IgG2ab+ B cells, 2) IgG2ab+ B cells with impaired Fas expression were also subjected to suppression by WT Igha/a T splenocytes, and 3) the suppression establishment was totally inhibited when both Pfp- and Fas-dependent mechanisms were simultaneously blocked, i.e., when Igha/a Pfpo/o T cells were used to induce suppression against Ighb/b Faslpr/lpr B cells. These results provide the first demonstration of the existence of alternative or simultaneous use of the major cytotoxic mechanisms in a T cell-mediated down-regulation of an Ig production.

Inhibition of IgG2ab production by Ig allotype-specific T cells can Be mediated without T-B cell contact

The growth of IgG2ab-producing CB101 myeloma cells, subcutaneously or intraperitoneally inoculated into histocompatible BALB/c Igha mice sensitized against this Ig allotype, was delayed by 2-4 weeks compared to normal mice. While IgG2ab production was detected in the sera of 75-100% of normal mice, it was irreversibly inhibited in 100% of sensitized mice. IgG2ab suppression (IgG2ab sup) was also systematically obtained in sensitized but not normal recipients, implanted ip with a 0.1-micrometer-pore diffusion chamber (DC) containing CB101 cells. This time, the specific IgG2ab sup was reversible in vitro in the absence of anti-IgG2ab T cells. Adoptive transfer, of unfractionated or T but not B splenocytes from their sensitized counterparts into normal mice, 1 day before DC implantation, induced IgG2ab sup as well. These results indicate that, in these experimental circumstances, IgG2ab sup can also be mediated by diffusible suppressive factors produced by the effector T cells, without direct T-B-cell contact.

The T/B cell interaction involved in induction of the mouse IgG2ab suppression is restricted by major histocompatibility complex class I, but not class II molecules

To determine the major histocompatibility complex (MHC) restriction of the T/ B cell interaction involved in a negative regulation of Ig production, we used mouse model of T cell-induced IgG2ab suppression in vivo. Normal or specifically triggered T splenocytes from mice of the Igha haplotype, when neonatally transferred into histocompatible Igha/b heterozygotes, are able to induce a specific and total suppression of the IgG2ab allotype. Nevertheless, only transfer of IgG2ab-primed Igha T splenocytes induces this suppression in Ighb/b homozygous congenic mice in which the whole IgG2a isotype production is inhibited. This suppression is chronically maintained by CD8+ T cells, but can be experimentally reversed. We have established that the suppression induction required a CD4+CD8+ T cell cooperation and operated via the recognition by the involved TCR of C gamma 2ab-derived peptides presented by the target B cells in an MHC haplotype-restricted manner. Here, by using Ighb mice genetically deficient for MHC class I (beta 2-microglobulin%, or beta 2m%) or class II (I-A beta%) molecules, we demonstrate functionally that the suppression induction implicates an MHC class I-, but not class II-restricted interaction. Indeed, the anti-IgG2ab T cells transferred into Ighb H-2b I-A beta% mice carry out the suppression process normally, while in Ighb H-2b beta 2m% recipients, their suppression induction capacity is significantly inhibited. Moreover, the C gamma 2ab 103-118 peptide, identified as the sole C gamma 2ab-derived peptide able to amplify the anti-IgG2ab T cell reactivity in Igha H-2b mice, is also able to stabilize the H-2Db, but not the H-2Kb class I molecules at the surface of RMA-S (TAP2-, H-2b) cells. These results indicate that, despite the CD4+/CD8+ T cell cooperation during the induction phase of suppression only MHC class I molecule expression is required at the surface of IgG2ab+ B cells for suppression establishment.