CD45 expression by murine B cells and T cells: Alteration of CD45 isoforms in subpopulations of activated B cells

The CD45 family of high molecular weight cell surface glycoproteins is abundantly expressed by virtually all hematopoietic cells. CD45 molecules exist as multiple isoforms whose extracellular portions vary in protein structure and carbohydrate content but whose intracellular portions are highly conserved and possess tyrosine phosphatase activity. In this review we summarize current studies describing CD45 isoform expression on peripheral and thymic lymphocytes. Further, we analyze changes in CD45 isoform expression by selective populations of activated B cells.     

CD4+ T cells clear virus but augment disease in mice infected with respiratory syncytial virus. Comparison with the effects of CD8+ T cells.

Respiratory syncytial (RS) virus-specific T cell lines were derived from the spleens of BALB/c mice primed by intranasal infection with RS virus. The lines were expanded by repeated antigenic stimulation in vitro, and separated into CD4+ and CD8+ T cell-enriched fractions by immunomagnetic adhesion. The effects of passive transfer of these fractions into RS virus infected mice were observed. The most severe immunopathological changes were seen in mice receiving CD4+ cells. Transfer of CD4+, CD8+ or both cell fractions caused RS virus-infected mice to become ill and lose weight. Both cell lines caused an increase in the severity of lung pathology (as monitored by bronchoalveolar lavage) with the appearance of lung haemorrhage and polymorphonuclear cell efflux. In addition, recipients of CD4+ cells developed striking pulmonary eosinophilia. In CD4+ cell recipients, 5 x 10(5) cells were sufficient to decrease lung virus titre, whereas 2 x 10(6) CD8+ cells were needed to produce a similar effect. The unseparated T cell line and the CD4+ cell fraction secreted significant amounts of IL-3, IL-4 and IL-5 (P less than 0.001). High levels of IL-2 were produced only by the unseparated T cell line. The CD8+ cell fraction secreted IL-3 only. The results show that, cell-for-cell, CD4+ cells are more anti-viral and more immunopathogenic than CD8+ cells in RS virus infected mice. Such effects may have contributed to the augmented disease seen in some infants vaccinated against RS virus.     

Basophils inhibit proliferation of CD4+ T cells in autologous and allogeneic mixed lymphocyte reactions and limit disease activity in a murine model of graft versus host disease

Basophils are known to modulate the phenotype of CD4⁺ T cells and to enhance T helper type 2 responses in vitro and in vivo. In this study, we demonstrate that murine basophils inhibit proliferation of CD4⁺ T cells in autologous and allogeneic mixed lymphocyte reactions. The inhibition is independent of Fas and MHC class II, but dependent on activation of basophils with subsequent release of interleukin‐4 (IL‐4) and IL‐6. The inhibitory effect of basophils on T‐cell proliferation can be blocked with antibodies against IL‐4 and IL‐6 and is absent in IL‐4/IL‐6 double‐deficient mice. In addition, we show that basophils and IL‐4 have beneficial effects on disease activity in a murine model of acute graft‐versus‐host disease (GvHD). When basophils were depleted with the antibody MAR‐1 before induction of GvHD, weight loss, GvHD score, mortality and plasma tumour necrosis factor levels were increased while injection of IL‐4 improved GvHD. Basophil‐depleted mice with GvHD also have increased numbers of CD4⁺ T cells in the mesenteric lymph nodes. Our data show for the first time that basophils suppress autologous and allogeneic CD4⁺ T‐cell proliferation in an IL‐4‐dependent manner.     

CD23 defines two distinct subsets of immature B cells which differ in their responses to T cell help signals.

Transitional immature B cells undergo apoptosis and fail to proliferate in response to BCR cross-linking, thus representing a target for negative selection of potentially autoreactive B cells in vivo. In agreement with recent reports, transitional B cells were divided into developmentally contiguous subsets based on their surface expression of CD23. When transferred, CD23(+) transitional B cells readily localized to the splenic follicles and the outer PALS. Compared with CD23(-) transitional B cells, CD23(+) transitional B cells proliferated more vigorously and were rescued from BCR-induced apoptosis to a greater degree, by T cell help signals. However, both CD23(-) and CD23(+) transitional B cells failed to up-regulate CD86 (B7-2) in response to BCR ligation. These findings demonstrate that phenotypically defined subsets within the transitional B cell population are functionally distinct. Specifically, responsiveness to T cell help is a late acquisition corresponding to the stage when the B cells gain access to peripheral compartments enriched in antigen and activated T cells. The failure of transitional B cells to up-regulate CD86 to BCR-mediated stimulation suggests a unique interaction between transitional B cells and T cells with implications for tolerance in the T cell compartment.     

In vitro polyclonal activation of conventional T cells with a CD28 superagonist protects mice from acute graft versus host disease

Upon transplantation of T cells from a CD28 superagonist (CD28‐SA) treated donor into an irradiated allogeneic host, the CD28‐SA‐induced activation and expansion of T_reg cells inhibits acute graft versus host disease (aGvHD), while not abrogating the desired graft versus tumor effect. Human peripheral blood CD4⁺ T cells, however, harbor only very few T_reg cells. Therefore, we studied whether polyclonal in vitro prestimulation of conventional, that is T_reg‐cell‐depleted, CD4⁺ T cells of C57BL/6 mice with CD28‐SA‐coated paramagnetic beads is sufficient to protect recipient BALB/c mice from aGvHD. CD28‐SA prestimulation of conventional CD4⁺ T cells efficiently protected BALB/c recipient mice from aGvHD and CD28‐SA‐stimulated CD4⁺ and CD8⁺ T cells were capable of mediating long‐term protection from the BCL₁ lymphoma. The recently completed successful phase I testing of the human CD28‐SA TGN1412/TAB08 should greatly facilitate further development of this straightforward method into a novel immunotherapy for patients.     

Preferential expansion of Ly-1 B and CD4 CD8 T cells in the polyclonal lymphocyte responses to murine T.cruzi infection

Acute murine infection with T.cruzi results in polyclonal lymphocyteresponses manifested by blast transformation of a large fractionof B, CD4+, and CD8+ cells. We describe here the finding ofsignificant increases in the splenic representation of minorpopulations, Ly-1+ B cells and CD4-CD8- T cells. These lymphocytepopulations might play an important role in the host response,as shown by T.cruzi infection of hosts that had been lethallyirradiated and reconstituted with autologous bone marrow. Underthese conditions, the splenic polyclonal PFC responses are nearlyabrogated, and not restored by the transfer of syngeneic peritonealcells which, however, reconstitute T15 idiotype production inthe same hosts. Control levels of PFC responses, however, arereconstituted by transfer of syngeneic splenic T cells. Sincebone marrow-reconstituted animals contain normal numbers ofCD4+ and CD8+ T cells which are actually activated by infection,these results suggest the participation of other T cell populationsin the host response to infection, as also suggested by themarked increases in T cell receptor and messages detectedin the spleen of infected animals. The implications of thesefindings in immunopathology of Chagas' disease are discussed.     

Donor CD8 T cell activation is critical for greater renal disease severity in female chronic graft-vs.-host mice and is associated with increased splenic ICOS host CD4 T cells and IL-21 expression

Lupus-like renal disease in DBA/2-into-F1 (DBA → F1) mice is driven by donor CD4 T cells and is more severe in females. Donor CD8 T cells have no known role. As expected, we observed that females receiving unfractionated DBA splenocytes (CD8 intact → F1) exhibited greater clinical and histological severities of renal disease at 13 weeks compared to males. Surprisingly, sex-based differences in renal disease severity were lost in CD8 depleted → F1 mice due to an improvement in females and a worsening in males. CD8 intact → F1 female mice exhibited significantly greater donor and host effector (CD44^hi, CD62L^lo) CD4 T cells and ICOS^hi CD4 T follicular helper cells than males. CD8 depleted → F1 female mice exhibited a reduction in the absolute numbers of host, but not donor CD4 Tfh cells and lost the significant increase in host CD4 effector cells vs. males. Greater female IL-21 expression, a product of Tfh cells, was seen in CD8 intact → F1 and although reduced was still greater than male CD8 depleted → F1 mice. Thus, donor CD8 T cells have a critical role in mediating sex-based differences in lupus renal disease severity possibly through greater host ICOS^hi CD4 T cell involvement.     

Oral tolerance to haptens: intestinal epithelial cells from 2,4-dinitrochlorobenzene-fed mice inhibit hapten-specific T cell activation in vitro

The mechanisms underlying the induction of immunological tolerance after feeding soluble exogenous antigens, including proteins and haptens, are still unclear. Using a model of oral tolerance to the contact-sensitizing hapten 2,4-dinitrochlorobenzene (DNCB), we have compared the ability of intestinal epithelial cells and of Peyer's patch APC to present DNCB in vitro or ex vivo after oral feeding, to specific peripheral lymph node T cells from DNCB-sensitized mice. In contrast to Peyer's patch APC, which induce efficient hapten-specific T cell activation upon exposure to the hapten either in vitro or in vivo, mature MHC class-II-positive intestinal epithelial cells were unable to induce T cell activation in either case. Interestingly, enterocytes from DNCB-fed mice exerted a dramatic inhibitory effect on the proliferative response of hapten-primed T cells in response to dinitrobenzene sulfonate presented by syngeneic spleen cells. This inhibitory effect, which was also observed with supernatant of intestinal epithelial cells from DNCB-fed mice, could be reversed by neutralizing anti-transforming growth factor (TGF)-β antibodies. In addition, pre-incubation of hapten-sensitized T cells with enterocytes from DNCB-fed mice induced T cell anergy, which could be reversed by exogenous interleukin-2 or interleukin-4. These data demonstrate that intestinal epithelial cells activated in vivo by oral administration of DNCB are able to block proliferation of activated T cells through secretion of immunosuppressive cytokines such as TGF-β. It is proposed that intestinal epithelial cells may play a significant role in oral tolerance by limiting T cell-mediated hypersensitivity responses.     

Differential contribution of the FcRγ chain to the surface expression of the T cell receptor among T cells localized in epithelia: analysis of FcRγ-deficient mice

The function of the Fc receptors γ chain (FcR_γ) for the expression of the T cell receptor (TCR) complex and for T cell development, especially for T cells localized in epithelia, was investigated by analyzing FcR_γ-deficient mice. In wildtype mice, CD8αα⁺β^?TCRαβ⁺ T cells of intestinal intraepithelial lymphocytes (i-IEL) utilized CD3ζ homodimers and ζ-FcR_γ heterodimers, whereas CD8α α⁺β^?TCR_γδ⁺ i-IEL used ζ-FcR_γ and FcR_γ homodimers in the TCR complex. On the other hand, these T cells in FcR_γ-deficient mice contained only ζ homodimers. The surface expression of the TCR complex was reduced in CD8αα⁺β^?i-IEL and dendritic epidermal T cells (DETC) in these mice, whereas the development of these T cells was normal. The degree of reduction appeared to depend on the expression level of FcR_γ. In contrast to these populations, TCR_γδ⁺ intraepithelial T cells in reproductive organs (r-IEL) were dramatically decreased, suggesting that the development of r-IEL is FcR_γ-dependent, probably due to the predominant usage of FcR_γ homodimers in the TCR complex. These results indicate that the FcR_γ chain contributes differently to the TCR expression and to the development of T cells localized in epithelia.     

Detection of T cells reactive to intestinal alkaline phosphatase, an autoantigen in acute bacterial infections, and discrimination between autoantigen-specific CD5+ and CD5− B cells

Autoantibodies of the IgG isotype, specifically directed against intestinal alkaline phosphatase (IAP), occur transiently in the majority of sera from patients with acute bacterial infections. Sometimes they are observed in autoimmune diseases. Using a T cell proliferation assay, it was found that isolated peripheral blood mononuclear cells (PBMC) from IAP autoantibody (IAPA)-positive patients (n=18) responded significantly to IAP, whereas proliferation could not be induced in PBMC from healthy donors (n=11). Significant stimulation of PBMC from patients (n=11) was not obtained by use of transferrin, a common autoantigen in humans, indicating the specificity of stimulation of IAP-reactive T cells. Furthermore, T cell proliferation was observed when a highly purified IAP fragment (CNBr 21) spanning amino acids 334–462 of the primary structure of IAP was used as antigen. Thus, it was shown that an immunodominant T cell epitope resides within the CNBr 21 fragment which also contains a discontinuous B cell epitope as evaluated previously. Double immunocytochemical staining of T cell-depleted PBMC with IAP and an anti-human CD5 antibody allowed the detection of CD5⁺ B lymphocytes, which probably produce natural IAPA (nIAPA). These nIAPA-specific CD5⁺ B cells occurred with approximately the same frequency among T cell-depleted PBMC from healthy donors and those from patients. In contrast, IAPA-producing CD5⁻ B cells were found in B cell-enriched preparations from patients, but not in those from healthy individuals.