T cell-dependent B cell activation

Receptors for innate pathogen defence in insects are normal activation receptors for specific immune responses in mammals

The Toll genes code for activation receptors for innate microbial defence in insects and have recently been found to code for the Lps receptor in murine B lymphocytes. In this study, I will present evidence to show that this family of receptors are responsible for induction of specific thymus-independent immune responses in mice. T cells do not possess these pattern-recognizing receptors, but they possess structures capable of delivering activation signals to these receptors on B cells.

In vivo administration of interleukin-2 turns on anergic self-reactive T cells and leads to autoimmune disease

One major mechanism of self tolerance involves the deletion of T cell clones in the thymus. In athymic mice, tolerance to self antigens must be generated extrathymically. T cells with self-reactive receptors undergo either peripheral clonal deletion or become unresponsive (i.e. anergic). The unresponsive state of human and mouse T cell clones in vitro can be reversed by the addition of exogenous interleukin (IL)-2, thus transforming anergic T cells to an activated state. Here it is shown that the in vivo delivery of IL-2 to athymic BALB/c nu/nu mice abrogates the anergic state of self-reactive V beta 3+ and V beta 11+ T cells [which are normally deleted in the minor lymphocyte stimulatory (Mls)-1b-, I-E(+)-expressing euthymic counterparts]. Thus, V beta 3+ and V beta 11+ T cells from IL-2-treated nude mice proliferate in response to T cell receptor cross-linking and acquire effector functions as measured by their ability to deliver aid to B cells upon specific stimulation. This activation correlates with the development of autoimmune manifestations (DNA autoantibodies, rheumatoid factors, erythroleukopenia and minimal change nephritis) in these IL-2-treated mice.

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.

The B-cell activation pathway in human systemic lupus erythematosus: imbalanced in vitro production of lymphokines and association with serum analytical findings

Recent knowledge of B-lymphocyte physiology has clarified the role of T cell-derived lymphokines in clonal proliferation and differentiation of B-cell responses. Lymphokine production was analyzed in 19 systemic lupus erythematosus (SLE) patients and sex- and age-matched controls in relation to clinical activity and steroid treatment. When in vitro production of interleukin-2 (IL-2) and B-cell growth factor (BCGF) was tested, both activities were found to be diminished in the group of patients (P less than 0.01), while B-cell differentiation factor (BCDF) activity was higher in this group with respect to normal controls (P less than 0.01). Interestingly enough, this in vitro BCDF synthesis was positively correlated with clinical activity regardless of low-dose steroid treatment. A correlation was also found between BCDF production and the levels of IgG (r = 0.64, P less than 0.01), anti-DNA antibodies (r = 0.52, P less than 0.05), and the IgG/IgM ratio (r = 0.7, P less than 0.01) in serum. Implications of these abnormal T-lymphocyte functions in SLE with respect to in vivo B-cell function are discussed.

In vitro IgM rheumatoid-factor production induced by tetanus toxoid

In vitro stimulation of lymphocytes from healthy donors with well-defined polyclonal B cell activators may elicit the production of rheumatoid factor (RF) as well as other autoantibodies. Antigen stimulation may also result in polyclonal B cell activation, but it is not known if RF production is a feature of this response. Therefore, peripheral blood mononuclear cells from 36 healthy volunteers previously immunized to tetanus toxoid (TT) were stimulated in vitro for 9 days with a conventional antigen, TT, or pokeweed mitogen, a standard polyclonal B cell activator. Culture supernatants were analyzed for total IgG, total IgM, and IgM RF by ELISA. TT-induced IgM RF production was observed in 10/36 experiments compared to 18/36 experiments in which cells were cultured with pokeweed mitogen, with a similar magnitude of response to these respective stimulants. The in vitro IgM-RF response to TT did not require a recent in vivo TT booster immunization and was observed at an antigen does that elicits polyclonal B cell activation but not IgG specific anti-TT antibody. TT-induced IgM RF responder cultures demonstrated higher levels of total IgG and total IgM production than cultures not secreting IgM RF in response to TT. These results indicate that IgM RF production is a concomitant of the polyclonal B cell response elicited by a conventional antigen. Unlike other model systems, this antigen-induced RF response was not mediated by the action of IgG antibody containing immune complexes.

Adjuvant effects of orally administered saponins on humoral and cellular immune responses in mice

We have described adjuvant effects of orally administered Quillaja saponins on the immune responses of mice fed inactivated rabies antigen (AG). The in vivo lymphocyte proliferation in mice fed antigen + saponin (AG + SAP) was significantly greater than that in mice fed antigen (AG) alone. Further, the mitogen-induced cell proliferative responses in animals primed with AG + SAP was markedly increased compared with those in the AG group. These changes in clonal expansion were associated with an enhanced helper T cell (Th) and B cell co-operation. The in vivo cell proliferation and in vitro mitogen-induced responses of mice fed AG + SAP correlated with enhanced antibody synthesis. In mice fed saponin alone, there were significant increases in clonal expansion and lymphocyte function. Our present data indicate that the immunocompetence in animals fed AG + SAP was indeed evoked by saponins. Cytotoxic T lymphocyte activity in mice fed SAP or AG + SAP was detected 7 days after booster, in contrast to 21 days in mice fed AG alone. The natural killer cell activity in mice fed SAP alone was greatly enhanced and persisted for an extended period of time.

The host component of the popliteal lymph node graft-versus-host reaction. Selective representation of lymphocyte subsets and the requirement for alloantigenic incompatibility between donor cells and activated host B cells

We have examined the cellular changes taking place in rat popliteal lymph nodes undergoing a graft-versus-host (GvH) reaction. Examination of immunoperoxidase-stained lymph node sections, using a panel of mouse monoclonal antibodies directed against different rat lymphoid cell subsets, revealed a disorganization of the lymph node architecture with disappearance of the follicles, and an intermingling of T and B cells, so that no distinct T- and B-cell areas were visible any more. Since the GvH nodes showed a preferential accumulation of host B cells over host T cells (particularly over the W 3/25+ T helper cell subset), we also investigated the requirements for host B cell activation. The popliteal lymph node GvH reaction was induced in (PVG X DA)F1 rats by the injection of PVG cells into one foot and by DA cells into the other foot, and then immunoglobulin kappa allotype marked PVG B cells from athymic donors were injected intravenously. The allotype marked B cells proliferated vigorously in response to the DA T cells, but much less in response to the PVG T cells. These results indicate that the massive B-cell activation taking place in GvH reactions may require an alloantigen incompatibility between donor T cells and host B cells, and argue against non-specific mitogenic induction of the B cells.

The role of immunoglobulin receptors in "cognate" T-B cell collaboration

The functional effects of anti-Ig antibodies have been investigated, using an experimental system where B cell activation is brought about by direct and specific interactions with T helper (Th) cells without participation of surface Ig receptors on the responding B cell. We have used Th cell lines and clones directed to class II major histocompatibility complex antigens of the responding B cells, and titrated into cooperative cultures either purified rabbit anti-mouse mu, or monoclonal mouse anti-delta antibodies. Both types of antibodies greatly enhanced B lymphocyte responses to suboptimal concentrations of functionally efficient Th cells, while they had no effect in cultures containing optimal Th:B cell ratios. In contrast, helper activity by low efficiency Th was, at all Th:B cell ratios, enhanced by appropriate concentrations of anti-Ig antibodies. Anti-Ig effects were exclusively observed when B cells were the targets for "cognate" recognition by Th cells. We conclude that ligand binding to surface Ig receptors on resting B cells fails, in our experimental conditions, to overcome "linked" collaboration, but it greatly facilitates productive Th-B cell interactions. Whatever the mechanisms underlying this facilitation, the observations imply roles of surface Ig in Th-dependent B lymphocyte activation other than either passive "focusing" of antigen or activation into reactivity to soluble, unspecific factors.

The popliteal lymph node graft-versus-host (GvH) reaction in the rat: a useful model for studying cell interactions in the immune response?

Simonsen (1962), in his comprehensive review article on GvH reactions, rightly emphasized the great applicability this reaction had in immunological research. A broader implication of GvH reactions than was perhaps expected was evident when the complex interactions between the donor cells the host's immune system were detected. In this review article, one aspect of the donor-host cell interactions is discussed. Using the popliteal lymph node GvH reaction as a model, it has been shown that the hyperplastic response in the lymph node is due to an accumulation and proliferation of cells almost exclusively of host origin. B cells are preferentially activated, and although mitogenic factors are released into the circulation during systemic GvH reactions, the interaction between donor TH cells and host B cells is probably of a more specific nature, involving only those B cells expressing MHC antigens to which the donor cells are able to react. This will presumably lead to a panclonal activation of host B cells with antibody production against a wide variety of antigens. Athymic nude rats are often surprisingly resistant to GvH reactions induced by allogeneic T cells, and this resistance can now be satisfactorily explained in terms of natural resistance mechanisms. Sometimes F1 hybrid rats are also resistant to GvH reactions induced by parental cells, indicating that this resistance may be restricted by non-codominantly inherited genes.

A self-Ia reactive T cell clone directly stimulates every hundredth B cell and helps antigen-specific B cell responses

A self-I-A reactive T cell clone has been isolated that proliferates in the presence of irradiated syngeneic spleen cells in mouse serum. The cells directly stimulate approximately every hundredth B cell, which includes small B cells, into both proliferation and antibody formation in the absence of added antigen. There is evidence to suggest that cells of this type are generally present in murine T cell populations. The cells may recognize self-I-A as such. We failed to obtain evidence, but cannot exclude that they recognize an antibody idiotype in addition. The clone also augments antigen-specific antibody responses in vivo and in vitro in the presence of antigen. In the in vitro response to a T-independent TI-2 antigen, the T cells in co-culture with B cells induced specifically the production of IgG1 antibodies, thus mimicking the function of major histocompatibility complex-restricted, antigen-specific helper T cells. However, IgG1 production was not observed when splenic T cells were added to this system. Thus, self-I-A reactive T cells, generally inactive in the system, may locally help B cell responses to antigens which the T cells do not themselves recognize. In such instances one might attribute to the T cells antigen or idiotype-binding specificities which the cells in reality do not display.

B-cell growth factor (B-cell growth factor I or B-cell-stimulating factor, provisional 1) is a differentiation factor for resting B cells and may not induce cell growth

B-cell growth factor I [BCGF I or B-cell-stimulating factor, provisional 1 (BSFp1)] has been defined as a T-cell-derived lymphokine that acts as a co-stimulator of polyclonal B-cell growth in B cells cultured with anti-mu, anti-delta, or anti-Ig. Based on a number of studies it has been suggested that anti-Ig induces cell enlargement, entry into the G1 phase of the cell cycle, and expression of receptors for BSFp1. BSFp1 then induces entry of the cells into S phase. By adding BSFp1 prior to anti-Ig, we have found evidence that BSFp1 renders cells susceptible to anti-Ig-mediated entry of cells into G2/S phase. In contrast, if cells are first treated with anti-Ig, washed, and then cultured with BSFp1, they do not enter S phase. Taken together, these results suggest that BSFp1 acts on the resting B cells not as a growth factor but rather as a lymphokine that prepares cells for anti-Ig-mediated activation. Taken together with previous reports that BSFp1 induces increased expression of Ia antigens on resting B cells, these studies suggest that BSFp1 may be a differentiation factor rather than a growth factor and that it acts on resting B cells.

Cellular basis of immune response to antigens of ABO blood-group system. Capacity to provide help during response to T-cell-dependent ABO-system antigens is restricted to individuals of blood group O

It is postulated that the near-identity of the A and B antigens in man makes it impossible for individuals of groups A, A2, or B to provide T-cell help during a response to T-dependent non-self antigens by B2 lymphocytes. Thus, people of these groups are unable to produce immune IgG antibody. The presence of naturally occurring IgM antibody to non-self antigen from the age of 6 months is due to a separate system of T-cell-independent B1 cells which have arisen during evolution to protect the host against polysaccharide-encapsulated bacterial pathogens.

Lymphokines active in B cell proliferation and differentiation

Activation of B cells from a resting to an Ig-secreting cell is a multistep event regulated by T cells and their products. These steps include (1) induction of the clonally selected B cell, (2) growth, and (3) maturation of the proliferating cell to Ig secretion. In model systems, induction can be achieved either by direct signalling to the B cells by T cells recognizing surface structures without Ig involvement or, alternatively, via Ig only. Under physiologic conditions a cooperativity of the two events is assumed. Following induction, B cells are supposed to express receptors for growth and maturation factors produced by the neighboring T cell. Growth and differentiation are mediated by distinct molecules.

Antigen-specific helper T-cell clone supernatant is sufficient to induce both polyclonal proliferation and differentiation of small resting B lymphocytes

Gradient-purified resting B lymphocytes can be polyclonally stimulated by antigen-specific major histocompatibility complex (MHC)-restricted helper T lymphocytes as well as by antigen-activated helper T-cell supernatant. In contrast to what has been described so far, we show that helper T-cell supernatant (in the absence of any other added stimulus, such as that provided by anti-mu antibodies) is sufficient to induce both proliferation of resting B cells and their differentiation into IgM-secreting cells. The stimulation induced by the helper T-cell supernatant takes place in serum-free medium and is not MHC-restricted. Our findings strongly support the existence of a B-cell activating factor acting on the resting B cell and causing it to enter the G1 phase of the cell cycle in a MHC-unrestricted manner.