Concanavalin A inhibits the effector phase of specific cytotoxicity

Recent Insights into Glucose-Responsive Concanavalin A-Based Smart Hydrogels for Controlled Insulin Delivery

Many efforts are continuously undertaken to develop glucose-sensitive biomaterials able of controlling glucose levels in the body and self-regulating insulin delivery. Hydrogels that swell or shrink as a function of the environmental free glucose content are suitable systems for monitoring blood glucose, delivering insulin doses adapted to the glucose concentration. In this context, the development of sensors based on reversible binding to glucose molecules represents a continuous challenge. Concanavalin A (Con A) is a bioactive protein isolated from sword bean plants (Canavalia ensiformis) and contains four sugar-binding sites. The high affinity for reversibly and specifically binding glucose and mannose makes Con A as a suitable natural receptor for the development of smart glucose-responsive materials. During the last few years, Con A was used to develop smart materials, such as hydrogels, microgels, nanoparticles and films, for producing glucose biosensors or drug delivery devices. This review is focused on Con A-based materials suitable in the diagnosis and therapeutics of diabetes. A brief outlook on glucose-derived theranostics of cancer is also presented.

Concanavalin A induces a cytoskeletal association of T200 molecules in T lymphocytes

A recent report indicated that T200 molecules interact with elements of the cytoskeleton in BW5147 T lymphoma cells. We have confirmed the cytoskeletal association of T200 by examining nonionic detergent-soluble and detergent-insoluble fractions of murine T cell tumor cell lines, cloned cytotoxic T lymphocyte lines, and thymocytes. Concanavalin A (Con A)-treated and untreated cells were extracted with 0.5% Triton X-100 and the remaining insoluble material was extracted under conditions allowing actin depolymerization. In the absence of Con A treatment, little T200 could be recovered from the depolymerized insoluble fraction. However, in T cells treated with capping concentrations of Con A, a considerable amount of T200 was rendered insoluble in nonionic detergent, and T200 could be recovered from the insoluble fraction by a buffer which dissociates actin polymers. A lesser, but still significant, amount of T200 associated with the detergent-insoluble fraction of thymocytes treated with concentrations of Con A and succinyl Con A, which are mitogenic for T cells. We also found that in T cells treated with mitogenic concentrations of succinyl Con A, more T200 associated with cytoskeleton than did H-2 or LFA-1 molecules. Because T200 is such a predominant molecule on the surface of T cells, such translocations of the molecule may have a major impact on the physiology of the cell, especially if T200 functions as a protein tyrosine phosphatase as recent evidence by others suggests.

Non-specific propagation of human antigen-dependent T lymphocyte clones

Most T lymphocyte clones require periodical restimulation with specific antigen in addition to growth factors to maintain long-term growth in vitro. Since in many cases the specific antigen for a given T cell clone is not known or not available, alternative methods of stimulation are required. In this report a method for a non-antigen-specific stimulation of antigen-dependent proliferative or cytotoxic human T cell clones is described. Introduction of aldehydes into galactose residues on stimulator cells leads to the stimulation of T cell clones by modified stimulator cells in the absence of a mitogen. This system is as efficient as the use of mitogenic antibodies or lectins but avoids the disadvantages of these polyclonal T cell activators.

A compartment of effector helper and suppressor T cells in the normal mouse thymus

We have recently described an autonomously activated set of T cells in the spleens of normal and "antigen-free" mice which display effector T helper (TH) or T suppressor (TS) activities; we describe here an intrathymic effector T-cell compartment which directly helps or suppresses B-cell responses and appears to be distinct from the peripheral set of effector cells. Splenic effector T cells do not represent recent thymic migrants (because adult thymectomized mice have unaltered levels of effector TH and TS cells in the spleen), nor do intrathymic effector T cells represent circulating peripheral T cells (since thymic effector T cells are B2A2+, while splenic effector T cells are B2A2-). Furthermore, effector TH cells within the two compartments exert differential effector activities: splenic effector TH cells induce B cells to both proliferation and maturation, while thymic effector TH cells are defective in activating B-cell maturation. The present findings extend our studies on "natural" lymphocyte activities in the normal immune system, revealing the existence of two apparently distinct effector T-cell compartments. The potential significance of the intrathymic set of effector cells in repertoire selection is considered.

Inhibition of T cell cytolytic potential by concanavalin A: a result of activation?

We investigated the effects of concanavalin A (Con A) on T cell-mediated lympholysis. Human cytotoxic T cell lines were generated from peripheral blood and these lines were shown to lyse lectin-coated K562 target cells. Addition of soluble Con A to the assay resulted in a dose-dependent inhibition of the cytolysis. Preincubation experiments demonstrated that this inhibitory effect was exerted at the effector cell level. F(ab')2 fragments of WT32, a monoclonal antibody against T3, blocked the cytolysis of Con A-preincubated K562 target cells to a large extent. We further showed that Con A strongly inhibited the cytolysis exerted by alloantigen-specific, major histocompatibility complex (MHC)-restricted cytotoxic T cell lines against their specific targets. On the other hand, Con A had no clear inhibitory effect on the cytotoxicity of freshly isolated peripheral blood mononuclear cells against K562 target cells. We hypothesize that Con A-induced inhibition of cytotoxicity may be explained by a direct triggering of the lytic potential of activated T cells.

Activation of Lyt-2+ T cells by antibodies towards brain-associated antigens. II. Antibody-dependent induction of "nonspecific" cell-mediated cytotoxicity

Purified IgG from a rabbit anti-mouse brain antiserum (RaMB), previously shown to activate most Lyt-2+ T cells, was probed for lectin-like effects on cell-mediated cytolysis (CML) and found to induce "nonspecific" killing of syngeneic B cell targets by allospecific cytotoxic T lymphocytes (CTL) as effectively as concanavalin A, and at levels comparable to antigen-specific cytolysis of target cells. Interestingly, RaMB-mediated "nonspecific" CML of syngeneic targets, by polyclonally or specifically activated CTL, was restricted to B cell targets and cold target inhibition experiments indicated that syngeneic target cell blasts do not functionally interact with effector CTL in the presence of RaMB. The role of target cell Fc receptors was demonstrated by the competitive inhibition of RaMB-dependent CML by normal rabbit IgG. We conclude that RaMB both activates and bridges the effector CTL to target cells, RaMB-mediated "nonspecific" CML being a similar phenomenon to lectin-facilitated "nonspecific" cytolysis, and the mirror image of classical ADCC. These observations allowed us to interpret the effects of RaMB on allo-major histocompatibility complex-specific CML. Since RaMB stimulates specific CML of B cell targets while selectively blocking that of T cell targets, we conclude that RaMB antibodies interact with structures associated with the CTL antigen receptor, explaining the contradictory effects previously reported with monoclonal antibodies against the CTL receptor complex, which can both stimulate and inhibit T cell functions.

The appearance of cytotoxic lymphocytes in unstimulated and concanavalin A-activated human peripheral mononuclear cells

The presence of a mitogenic lectin during the cytotoxicity assay enables the detection of non-specific cytotoxicity (lectin-dependent cell-mediated cytotoxicity (LDCC]. Human mononuclear cells cultured at 37 degrees C in vitro became cytotoxic after 1 day, as detected by the LDCC. In some individuals, the presence of adherent cells during culture inhibited the development of cytotoxicity. Activation by concanavalin A (Con A) resulted in enhanced cytotoxicity after a few days in culture. There was no clear relationship between the lectin doses required for optimal induction of cytotoxicity and [3H]thymidine uptake. High Con A concentrations that induced a proliferative response often inhibited cytotoxicity in mononuclear cells depleted of adherent cells.

The role of accessory cells in polyclonal T cell activation. III. No requirement for recognition of H-2-encoded antigens on accessory cells

Highly purified murine lymph node T cells were used to test the hypothesis that polyclonal T cell activation requires the recognition of mitogen-modified major histocompatibility complex (MHC) antigens on accessory cells (AC) by the T cells. A variety of tumor cells lines, including macrophage, B and mast cell tumors, as well as thymomas, were shown to function as AC in concanavalin A-induced T cell activation, even if they expressed only one class of MHC antigens or none at all. In contrast to antigen-specific responses, where the Lyt-2+ phenotype is reportedly associated with recognition of class I MHC antigens, T cells enriched for or depleted of Lyt-2+ cells were not preferentially activated in the presence of class I- or class II-positive AC, respectively. In addition, as shown by others in the guinea pig and in the rat systems, T cell proliferation induced by oxidation of cell surface sugars is equally effective if T cells or AC are oxidized. T cell mitogens, therefore, do not seem to act by altering MHC antigens on AC, but rather by providing T cell-AC contact via their agglutinating properties.

Effect of concanavalin A treatment on the allogeneic response of mice to challenge with P 815 mastocytoma: interleukin 2 treatment reverses concanavalin A suppression in vivo

Mice injected repeatedly with concanavalin A (Con A) prior to and following challenge with P 815 mastocytoma are suppressed in their cell-mediated cytotoxicity responses. Earlier studies showed that pretreatment of the animals with silica to affect macrophage (M phi) functions reversed the Con A suppression. In the present paper we have shown that peritoneal exudate cells (PEC) induced/activated by ip injection of Con A were able to transfer suppression to normal mice. Separation of the PEC populations into adherent and nonadherent cells abrogated their capacity to transfer suppression. It was further shown that Con A is not functioning in this in vivo system to block effector activity of cytotoxic cells on target cells, and PEC induced with Con A were not directly cytotoxic to target P 815 cells. Finally, we were able to show that the cytotoxicity response of Con A-suppressed mice could be enhanced by treatment with concentrated culture supernatants of normal mouse spleen cells, rich in interleukin 2 (IL 2) activity. Attempts to detect a recently described mouse serum inhibitor of IL 2 in normal or Con A-treated mice were unsuccessful and spleen cells from Con A-treated mice lost their capacity to generate IL 2 in vitro when cultured under appropriate conditions. Taken together, these results suggest that suppression of cell-mediated immune responses in Con A-treated mice results from interruption of the normal generation of IL 2 helper effects necessary for the activation of cytotoxic effector T cells in vivo.

alpha-Fucose inhibits human mixed-lymphocyte culture reactions and subsequent suppressor cell generation

Carbohydrate moieties serve as important sites of interaction for many lymphocyte activities. The potential role of saccharides in the cellular interactions involved in mitogen-, antigen-, and alloantigen-induced proliferation was investigated. Eight different monosaccharides were tested for their inhibitory potential when added to uni- and bidirectional mixed-lymphocyte culture (MLC) reaction as well as to mitogen (Con A, PHA, PWM)-stimulated cultures. Only alpha-L-fucose blocked the MLC reaction in a dose-dependent fashion while having no effect on mitogen stimulation, although antigen-specific stimulation was also blocked by fucose. Similarly alpha-L-fucose specifically inhibited the MLC-induced generation of suppressor cells. Pretreatment of the MLC responder cells with fucose dehydrogenase abolished the MLC reaction while stimulator cell pretreatment had no effect, suggesting that the recognition site of the former contained alpha-L-fucose. The generation and the effector phase of Con A-induced suppressor cells was not affected by fucose, indicating that different receptors are involved in the latter. Apparent competitive inhibition by exogenous fucose of the cell-cell interaction required for the MLC reaction suggested that this monosaccharide is an essential constituent of allogeneic recognition sites.

Induction of cytolytic T lymphocytes by mitogenic lectins is specifically inhibited by anti-Lyt-2 antibodies

The effects of anti-Lyt-2 antibodies at different levels of lectin-induced cytotoxic T lymphocyte (CTL) responses have been analyzed. Induction of interleukin 2 (T cell growth factor) reactivity ("step 1") by three different lectins was profoundly inhibited by anti-Lyt-2 antibodies. Also lectin-dependent cytotoxicity of polyclonally induced CTL was significantly inhibited by anti-Lyt-2 antibodies, whereas the interjacent IL2-dependent proliferation of "step 1"-induced CTL precursors was unaffected by these antibodies. The anti-Lyt-2-mediated inhibition was selective and specific to the target cell determinants and limited to the events in CTL responses which require interactions with the lectins. These results parallel those obtained in antigen-induced responses and indicate that antigens and lectins induce CTL responses by interacting with the same type of membrane receptor complex containing Lyt-2 molecules and antigen-specific receptors. It follows that T cell mitogenic lectins do not appear to deliver triggering signals through "mitogen" receptors.

Cytotoxic T-lymphocytes. How do they function?

The central theme of this work has been the roles of the CTL receptor and of MHC-proteins in CTL recognition and lysis. A major conclusion that may be deduced from the work presented here is that one CTL receptor is responsible for both target cell recognition and lysis. Although their function as recognitive structures is well established, involvement of MHC-proteins in the events that follow recognition has not been investigated in detail. We have proposed that MHC-proteins are molecular mediators whereby CTL receptors transmit signals ultimately leading to lysis of the target cell. I see future work on CTL-mediated lysis proceeding in the following directions: 1. Verification and analysis of the precise role of MHC proteins in CTL recognition and lysis by use of cell and vesicle systems of defined composition and structure. 2. Study of CTL-mediated 'lethal hit' in systems enabling analysis of early events (millisecond level) preceding lysis. 3. Grafting of CTL receptor(s) activity onto naive cells, using liposomes or other vehicles, and 4. Production of idiotypic reagents such as monoclonal antibodies specific for the combining site/effector mechanism of CTL.