T cell aggregation

Abstract: T cell activation is associated with a rapid increase in intracellular fructose-2,6-bisphosphate (F2,6BP), an allosteric activator of the glycolytic enzyme, 6-phosphofructo-1-kinase. The steady state concentration of F2,6BP in T cells is dependent on the expression of the bifunctional 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases (PFKFB1-4) and the fructose-2,6-bisphosphatase, TIGAR. Of the PFKFB family of enzymes, PFKFB3 has the highest kinase:bisphosphatase ratio and has been demonstrated to be required for T cell proliferation. A small molecule antagonist of PFKFB3, 3-(3-pyridinyl)-1-(4-pyridinyl)-2-propen-1-one (3PO), recently has been shown to reduce F2,6BP synthesis, glucose uptake and proliferation in transformed cells. We hypothesized that the induction of PFKFB3 expression may be required for the stimulation of glycolysis in T cells and that exposure to the PFKFB3 antagonist, 3PO, would suppress T cell activation. We examined PFKFB1-4 and TIGAR expression and F2,6BP concentration in purified CD3+ T cells stimulated with microbead-conjugated agonist antibodies specific for CD3 and the co-stimulatory receptor, CD28. We then determined the effect of 3PO on anti-CD3/anti-CD28-induced T cell activation, F2,6BP synthesis, 2-[1-14C]-deoxy-d-glucose uptake, lactate secretion, TNF-α secretion and proliferation. Finally, we examined the effect of 3PO administration on the development of delayed type hypersensitivity to methylated BSA and on imiquimod-induced psoriasis in mice. We found that purified human CD3+ T cells express PFKFB2, PFKFB3, PFKFB4 and TIGAR, and that anti-CD3/anti-CD28 conjugated microbeads stimulated a >20-fold increase in F2,6BP with a coincident increase in protein expression of the PFKFB3 family member and a decrease in TIGAR protein expression. We then found that exposure to the PFKFB3 small molecule antagonist, 3PO (1–10 μM), markedly attenuated the stimulation of F2,6BP synthesis, 2-[1-14C]-deoxy-D-glucose uptake, lactate secretion, TNF-α secretion and T cell aggregation and proliferation. We examined the in vivo effect of 3PO on the development of delayed type hypersensitivity to methylated BSA and on imiquimod-induced psoriasis in mice and found that 3PO suppressed the development of both T cell-dependent models of immunity in vivo. Our data demonstrate that inhibition of the PFKFB3 kinase activity attenuates the activation of T cells in vitro and suppresses T cell dependent immunity in vivo and indicate that small molecule antagonists of PFKFB3 may prove effective as T cell immunosuppressive agents.

Abstract: CD100 is a 150-kDa human surface glycoprotein implicated in T cell activation. Using BB18 and BD16 mAbs to two discrete epitopes of the CD100 molecule, we have shown previously that triggering the CD100 molecule through the BB18 epitope is comitogenic with PMA, whereas it lacks a detectable effect on CD2- and CD3-induced PBMC proliferation. Conversely, triggering the CD100 molecule through the BD16-defined epitope only exerts an effect on CD2- and CD3-induced PBMC proliferation. In the present study, we investigated the molecular relationship between CD100 and another surface structure involved in human T cell proliferation, the CD45 phosphatase. We show in this study that CD100 is associated with CD45 and that this association has functional significance. The association was demonstrated using coimmunoprecipitation and detection of CD45 enzymatic PTPase activity. Furthermore, we show that the association is increased during T cell activation and that triggering CD45 molecules through discrete epitopes induces the down-modulation of CD100 molecules at the cell surface. Interestingly, we demonstrate that this modulation could be attributed to the shedding of a soluble form of CD100 in the culture supernatant. Finally, one of the functional consequences of this T cell activation-induced CD100-CD45 association is revealed by the finding that CD100 mAbs have an effect on CD45-induced T cell aggregation.

Abstract: beta 2 integrin (CD11a,b,c/CD18)-mediated cell adhesion is required for many leukocyte functions. Under normal circumstances, the integrins are nonadhesive, and become adhesive for their cell surface ligands, the intercellular adhesion molecules (ICAMs), or soluble ligands such as fibrinogen and iC3b, when leukocytes are activated. Recently, we defined a peptide derived from ICAM-2, which specifically binds to purified CD11a/CD18. Furthermore, this peptide strongly induces T cell aggregation mainly mediated by CD11a/CD18-ICAM-1 interaction, and natural killer cell cytotoxicity. In the present study, we show that the same ICAM-2 peptide also avidly binds to purified CD11b/CD18, but not to CD11c/CD18. This binding can be blocked by the CD11b antibody OKM10. The peptide strongly stimulates CD11b/CD18-ICAM-1-mediated cell aggregations of the monocytic cell lines THP-1 and U937. The aggregations are energy and divalent cation-dependent. The ICAM-2 peptide also induces CD11b/CD18 and CD11c/CD18-mediated binding of THP-1 cells to fibrinogen and iC3b coated on plastic. These findings indicate that in addition to induction of CD11a/CD18-mediated cell adhesion, the ICAM-2 peptide may also serve as a "trigger" for high avidity ligand binding of other beta 2 integrins.

Abstract: We describe studies of a human leukocyte antigen, termed Leu-13, that is defined by a mouse monoclonal antibody that reacts with T and B lymphocytes. Optimal staining of T cells requires a much higher concentration of alpha Leu-13 (greater than or equal to 200 micrograms/ml) than alpha Leu-4 (6 micrograms/ml). However, the fluorescence intensity of T cells labeled with alpha Leu-13, washed and kept at 4 degrees C for different periods, does not diminish more rapidly than the fluorescence of T cells stained with alpha Leu-4 and treated the same way. These results indicate that the novel binding pattern of alpha Leu-13 is not due to weak affinity. Immunoprecipitation and SDS-PAGE analysis indicates that alpha Leu-13 precipitates a major 16-kilodalton (16kd) band and a minor 26kd component from Nonidet P-40-solubilized membrane of normal T cells that are surface-labeled with 125I. The similar sizes of the 16kd to 26kd (Leu-13) and 22kd to 28kd (Leu-4) molecules prompted us to compare the capacity of alpha Leu-13 and alpha Leu-4 to trigger T cell functions, and to co-modulate and co-precipitate the T cell antigen receptor. Unlike alpha Leu-4, alpha Leu-13 does not trigger T cell proliferation or augment NK activity, but inhibits the mitogenic effect of alpha Leu-4/T3 antibodies. alpha Leu-13 also induces T cells to clump into large aggregates after 16 hr of culture at 37 degrees C, but not at 4 degrees C. T cell aggregation is triggered by concentrations of alpha Leu-13 as low as 12.5 ng/ml. At this concentration, alpha Leu-13 is not detectable on the surface of T cells by indirect immunofluorescence analysis in a FACS. On leukemic T cells, alpha Leu-4 was found to co-modulate and co-precipitate a 38kd to 44kd dimer having apparent idiotypic determinants, whereas alpha Leu-13 did not. The possible significance of these findings to immune regulation by T cells is discussed.