Reciprocity in lymphocyte interactions

Blocking Superantigen-Mediated Diseases: Challenges and Future Trends

Superantigens are virulence factors secreted by microorganisms that can cause various immune diseases, such as overactivating the immune system, resulting in cytokine storms, rheumatoid arthritis, and multiple sclerosis. Some studies have demonstrated that superantigens do not require intracellular processing and instated bind as intact proteins to the antigen-binding groove of major histocompatibility complex II on antigen-presenting cells, resulting in the activation of T cells with different T-cell receptor Vβ and subsequent overstimulation. To combat superantigen-mediated diseases, researchers have employed different approaches, such as antibodies and simulated peptides. However, due to the complex nature of superantigens, these approaches have not been entirely successful in achieving optimal therapeutic outcomes. CD28 interacts with members of the B7 molecule family to activate T cells. Its mimicking peptide has been suggested as a potential candidate to block superantigens, but it can lead to reduced T-cell activity while increasing the host's infection risk. Thus, this review focuses on the use of drug delivery methods to accurately target and block superantigens, while reducing the adverse effects associated with CD28 mimic peptides. We believe that this method has the potential to provide an effective and safe therapeutic strategy for superantigen-mediated diseases.

Signals involved in germinal center reactions

Many of the features observed in the in vitro cultures discussed in this review coincide with characteristics described for an in vivo germinal center response. FDC and T cells are required to maintain B-cell proliferation which is confined to a finite amount of time (i.e. less than 2 wk). Large cellular aggregated form which contain many blasting cells undergoing DNA synthesis. In addition to proliferation, apoptosis is also occurring in the cultures but appears to be limited to the population which is not in contact with the FDC. The system can be driven by specific antigen, suggesting that clonal expansion is occurring. As in other immunological systems, there is an important role for adhesion molecules both for cluster formation and DNA synthesis. Antigen processing and presentation is a major event since blocking this through several mechanisms ends the stimulation. The role of T cells is essential both in vivo and in vitro; however, their exact contribution is still not well understood. It is interesting that blocking IL4 usage either by neutralizing the molecule or its receptor by monoclonal antibodies has no effect on the system. Which interleukins are important for germinal centers remains on open question. Evidence continues to accumulate on the important role of FDC and the molecules they express. Not only are the immune complexes an essential part, but it seems that molecules yet to be defined have an effect. For many practical reasons these have remained a mystery, but using our various systems we are attempting to reveal them. Two intriguing questions which remain include: 1. the molecular nature of the signalling between the FDC and B cell; and 2. how does the FDC retain the antigen in a native form for such long periods of time? An understanding of both mechanisms will provide us with a better appreciation for the events leading to a germinal center response and the immunological phenomenon referred to as memory.

Anti-Ig inactivation of the CH31 lymphoma model for immature B cell inhibits its ability to process pigeon cytochrome c

We wished to determine whether tolerized cells are able to process and present antigen based on our hypothesis that tolerized B cells be unable to function in the normal capacity as antigen-presenting cells if they are to remain the tolerant state. Results in this study show that the ability of the murine lymphoma model for immature B cells, CH31, to process pigeon cytochrome c was greatly down-regulated when cultured in the presence of rabbit anti-mouse IgM. In contrast, the same anti-IgM treatment had no significant effect on the antigen-presenting cell function of the lymphoma model for mature B cells, CH112. Presentation of CNBr-cleaved fragments of pigeon cytochrome c by either CH31 or CH12 cells was not affected by the antibody treatment. Furthermore, CH31 cells pre-incubated with pigeon cytochrome c were not subject to the anti-IgM inhibition of the antigen presentation. These observations suggest that pertubation of surface immunoglobulin molecules on CH31 immature B cells causes down-regulation of their antigen-processing machinery.

Differential activity of recombinant lymphokines on mouse B cell proliferation and cell cycle progression are revealed by 5-bromo-2'-deoxyuridine/Hoechst 33258 dye flow cytometry

Activation of resting mouse B cells with anti-mu chain antibodies (anti-mu) leads to cell proliferation. We have investigated the effect of recombinant T cell interleukins (IL 2 to IL 6) on such anti-mu-induced proliferation. No proliferative response was detected when IL 2, IL 3 and IL 6, either alone or in combination with anti-mu, were studied. Furthermore, neither IL 4 nor IL 5 could induce proliferation when added alone to B cell cultures. However, when combined with anti-mu, IL 4 as well as IL 5 stimulated cell growth. Analysis by 5-bromo-2'-deoxyuridine/Hoechst 33258 flow cytometry revealed distinct effects of IL 4 and IL 5 on B cell growth. In the presence of anti-mu, both IL 4 and IL 5 co-stimulated unfractionated splenic B cells. However, when B cells were separated into subpopulations by density, IL 4 proved to be a cell cycle progression factor, stimulating the majority of resting B cells to enter the cell cycle. In contrast, IL 5 had little effect on the resting fraction of B cells. Rather, IL 5 acted as a co-competence factor, stimulating predominantly low-density B cells. Following exposure of anti-mu alone, most B cells accumulated in the G1 of the second cycle. Upon addition of IL 4, the cells acquired the ability to progress into the next S phase compartment. Contrary to what is seen when B cells are stimulated by other mitogens, very few cells are in the G2 compartments after anti-mu plus IL 4 stimulation. This phenomenon was not due to a differential cell cycle progression rate. Our findings provide an analytical basis for fractionating cell-cycle-compartment-specific B cells for their molecular study.

Cognate interactions between helper T cells and B cells

The mechanism by which mammals produce an antibody response after exposure to antigen has intrigued biologists for over a hundred years. Here, Randolph Noelle and Charles Snow review some of the experimental findings since the early 1970s that have advanced understanding of the mechanisms operating during B-cell activation by thymus-dependent (TD) antigens. They also propose a model for B-cell activation that emphasizes the critical role played by direct cellular interactions between B cells and helper T(TH) cells and seek to place into perspective the role played by the membrane immunoglobulin (mlg) receptor in cognate responses.

An exact comparison between the efficiency of two- and three-cell-type clusters in mediating helper activity

Adoptive transfer of anti-H-2Db-primed B cells combined with anti-minor histocompatibility antigen-primed T cells enables the effect of linked and unlinked alloantigen to be compared. Over an extensive dose range the unlinked antigen proved ineffective. In a previously described, exactly comparable system for assessing the same effect on cytotoxic T cell precursors, low doses of linked antigen had proved approximately equally effective, but high doses of unlinked antigen had proved effective. This contrast supports the hypothesis that whereas T-B collaboration is mediated by two-cell-type clusters, collaboration between T cells requires formation of three-cell-type clusters.

The staphylococcal toxic shock syndrome toxin 1 triggers B cell proliferation and differentiation via major histocompatibility complex-unrestricted cognate T/B cell interaction

The Staphylococcus aureus exotoxin toxic shock syndrome toxin 1 (TSST-1) is a potent activator of T cells and monocytes. We have recently demonstrated that TSST-1 is a superantigen that binds monomorphic determinants on MHC class II molecules. In the present study, we have examined the effect of TSST-1 on the activation and differentiation of high density human tonsillar B cells. TSST-1 bound to tonsilar B cells with high affinity and saturation kinetics. This binding was effectively inhibited by a combination of anti-HLA-DR and anti-HLA-DQ mAbs. Treatment of purified B cells with TSST-1 failed to induce B cell proliferation or Ig production. However, in the presence of irradiated T cells, TSST-1 induced resting B cells to proliferate and differentiate into Ig secretory cells. TSST-1 mimicked nominal antigen in that its induction of B cell responses was strictly dependent on physical contact between T and B cells, and was profoundly inhibited by anti-MHC class II mAbs, anti-CD3 mAbs, and, to a lesser extent, by anti-CD18 mAbs. However, unlike nominal antigen, TSST-1-mediated T/B cell interactions were MHC unrestricted. These results suggest that TSST-1 induces T cell-dependent B cell proliferation and differentiation by virtue of its ability to mediate MHC-unrestricted cognate T/B cell interaction via the TCR/CD3 complex and MHC class II antigens.

Dynamics of complex formation between thymocytes and thymic medullary epithelial cells

Direct cell contact is an intrinsic part of several differentiation processes. A case in point is the formation of complexes between thymic lymphocytes and stromal cells, which are essential for T-cell maturation. The objective of the present work was to gain an insight into the mechanisms underlying the formation and dissociation of such lymphostromal complexes. Using an in vitro system, we show that the number of thymocytes adhering to a thymic medullary epithelial cell line (E-5) increases with time and reaches a plateau, after which some thymocytes spontaneously detach, while the rest remain attached to epithelium. The detached and forcibly removed thymocytes were analysed for their expression of L3T4 and Ly-2 antigens. The detached thymocytes showed a markedly lowered expression of both antigens. Both of these subtypes were shown to be totally refractory to form further complexes after a first encounter with E-5 cells. We also show that, once a mean adherence level of one thymocyte per E-5 cell was reached, (a) the level of further adherence increased exponentially, and (b) during this phase, binding of thymocytes to E-5 cells occurred in clusters, indicating pre-existing polarity or contact-induced polarization of the receptors at the E-5 cell surface.

Immune status of a mu, kappa transgenic mouse line. Deficient response to bacterially related antigens

We have examined the immune repertoire and immune response of a mouse that carries transgenes for a mu heavy chain and kappa light chain. The expression of these genes is under the regulation of their own controlling elements. The transgenes are expressed early in ontogeny and are easily detectable from day 13 of gestation onwards. The pre-B cells seem to function normally as they generate IgM-secreting colonies at normal frequencies. Colonies show predominantly the transgenic specificity. Expression of the transgenes is not limited to B cells since around 10%-20% of peripheral T cells and 50% of thymocytes express the mu transgene as an intracellular protein. Ectopic expression of kappa was not seen. The spleen size of the transgenic mouse is decreased by around 20%; this reduction is largely caused by a reduction of the B cell pool. Almost all B cells express the transgenes, only 30% co-express endogenous heavy chain genes and all co-express endogenous light chain genes. Serum Ig levels for IgM and IgA were normal, 20% of the IgM consist of the transgenic product. Serum IgG levels were decreased. T cell functions (helper and cytotoxic) were normal. Immune responses to conventional antigens were impaired, especially in the early phases of the immune response, but after boosting they were virtually normal, except for IgG3 which remained low. Primary antibody responses to T cell-independent antigens of the class II type (bacterially related antigens) were absent, although precursor frequencies for these antigens were within the expected range. The significance of this finding, as it relates to allelic exclusion of Ig genes, is discussed.

Anti-IgM-mediated B cell signaling. Molecular analysis of ligand binding requisites for human B cell clonal expansion and tolerance

The ligand binding requisites for membrane IgM-mediated signaling of human B lymphocyte clonal expansion and B cell tolerance were investigated with a well-characterized set of soluble murine anti-human IgM mAbs. Evaluation of the impact of mu chain domain specificity, affinity, and binding stoichiometry for membrane IgM on antibody-induced regulation of normal and leukemic B cell DNA synthesis revealed that the ligand binding requisites for inducing or, alternatively, suppressing B cell DNA synthesis are significantly different. First, while the induction of S phase entry required micrograms/ml concentrations of ligand, orders of magnitude lower concentrations of ligand sufficed for inhibitory signaling. Second, while an upper affinity threshold for achieving maximal stimulation of B cell DNA synthesis was never detected, inhibitory signaling by bivalent ligands appeared to become relatively affinity independent at Fab binding affinities greater than 7.0 x 10(6) M-1. Third, while a C mu 1-specific mAb with an enhanced incidence of monogamous binding to mIgM was ineffective at inducing B cell DNA synthesis, the antibody was not significantly compromised in ability to initiate inhibitory signals. These differences could be observed in a clonal B cell population which positively or negatively responded to mIgM ligation depending upon its state of activation. The accumulated observations indicate that the ligand binding requisites for inhibitory signal transduction in human B lymphocytes are much less rigorous than those for stimulatory signal transduction and suggest that many physiologically relevant anti-Ig antibodies are more likely to function in the negative feedback regulation of B cell responses than in the direct triggering of human B cell clonal expansion.

T helper cell-dependent induction of resting B cell differentiation need not require cognate cell interactions

We have analyzed the role of cognate interaction with helper T cells (Th) in support of resting B cell differentiation to plaque formation. Co-culture of histoincompatible resting B cells and resting Th cells resulted in the induction of plaque-forming cells when dimeric but not monomeric fragments of anti-T cell receptor (TcR) antibody were added to culture. The efficiency of B cell activation was comparable to that supported by lipopolysaccharide and lectin-mediated Th-B cell conjugate formation. Further, if resting Th cells were preactivated with antigen and histocompatible antigen-presenting cells, the requirement for addition of anti-TcR to mixtures of histoincompatible Th and B cells was obviated. These results demonstrate that TcR-mediated Th recognition of major histoincompatibility complex class II/antigen composites on the resting B cell membrane does not provide obligate signals for B cell differentiation to plaque formation. We are left with two possibilities. Either the entire process of Th cell-dependent induction of resting B cell differentiation is mediated by soluble lymphokines or if Th-B cell contact is mandatory, it is mediated through nonpolymorphic cell surface determinants.

A noncognate interaction with anti-receptor antibody-activated helper T cells induces small resting murine B cells to proliferate and to secrete antibody

Culture of small resting allogeneic B cells (of an irrelevant haplotype) with two clones of T helper (Th) cells that were activated by the F23.1 anti-T cell receptor antibody led to the activation of B cells to proliferate and to secrete antibody. Th cell supernatants by themselves had no effect on resting B cells (even in the presence of intact F23.1 antibody), but could induce antibody secretion by anti-Ig-preactivated B cells. Both F23.1+ clones (E9.D4 and 4.35F2) and one F23.1- clone (D2.2) could synergize with supernatants from activated E9.D4 T cells to induce B cell activation. F(ab')2 fragments of F23.1 induced E9.D4 to activate B cells as efficiently as intact F23.1 and B cell populations that had been incubated with F23.1 were not activated when cultured with E9.D4, although T cells recognized cell-presented F23.1 and were weakly activated. Reduction of the density of F23.1 adsorbed to plastic resulted in weak T cell activation, and these T cells did not induce B cell responses. Haptenated B cell populations, although recognized by E9.D4, were not activated. Separation of T and B cells by a 0.4-micron membrane prevented T-dependent B cell activation, although Th cell-derived B cell-activating lymphokines would be assayed across these membranes. These results suggest a polyclonal noncognate B cell activation that depends on physical contact between B cells and activated T cells. The requirement for a cognate interaction of Th with B cells for the production and delivery of B help can therefore be overcome by activating Th cells with high densities of T cell receptor ligands.

The molecular interactions with helper T cells which limit antigen-specific B cell differentiation

Helper T (Th) cell-dependent activation requirements for 2,4,6-trinitrophenyl (TNP)-specific resting B cells obtained from mice transgenic for Sp-6 mu, kappa genes were analyzed. Carrier-specific T cell help required linked recognition of TNP carrier and was functionally restricted by the B cell major histocompatibility complex. However, histoincompatible T cell-B cell conjugates formed by bridging surface immunoglobulin and Th cell receptor for antigen (TcR) through TNP-conjugated anti-TcR antibodies resulted in the efficient differentiation of TNP-specific B cells. Thus, Th cell-dependent cognate recognition of B cells is not obligatory. Specific conjugate formation could be obviated by using unconjugated fragments of anti-TcR antibodies. If dimeric, these fragments supported the Th cell-dependent differentiation of co-cultured histoincompatible resting B cells. Unconjugated monomeric fragments were ineffective, demonstrating the necessity for TcR cross-linking. Resting B cells from Sp-6+ mice rendered TNP-conjugated monomeric fragments of anti-TcR antibodies effectively multivalent, thereby satisfying conditions for the activation of co-cultured Th cells. The results demonstrate that Th cells do not transduce activation signals through TcR recognition of B cell membrane-associated ligand which limit the induction of B cell differentiation. Cross-linking of TcR on Th cells is required, sufficient and can be induced through interaction with the antigen-specific B cell surface.

Regulation of human B lymphocyte responsiveness

Regulation of human B cell responses is a complex process involving the activities of a variety of cells and cytokines. There are important differences between the regulation of human and murine B lymphocytes, especially with regard to the action of IL2 and IL4. In man, IL2 appears to play a central role in regulating B cell activation, proliferation and differentiation, whereas IL4 plays a regulatory role by inhibiting initial B cell activation. Above and beyond providing regulatory cytokines, activated human T cells can also directly stimulate B cell differentiation by a process that requires cellular contact. A better understanding of the nature of signal transduction during interactions between B cells and activated T cells and provided by the various cytokines should provide additional insight into the regulation of B cell function in man.