High-level IL-10 production by monoclonal antibody-stimulated human T cells.

We investigated interleukin-10 (IL-10) production in freshly isolated mononuclear cells and purified T cells in response to stimulation with monoclonal antibodies (mAb) recognizing CD3, CD2 and CD28, or with the bacterial products Staphylococcus aureus cells (SAC), staphylococcal enterotoxin (SEA) and lipopolysaccharide (LPS). IL-10 production was compared with that of IL-2, IL-4 and interferon-gamma (IFN-gamma). Similar to the other cytokines, in peripheral blood mononuclear cells (PBMC) from adult donors the highest IL-10 levels were produced in response to CD2 plus CD28 stimulation, within 72-96 hr of stimulation. Levels of IL-10 in response to CD2 plus CD28 stimulation (1.9 +/- 1 ng/ml) exceeded those in response to SEA (0.25 +/- 0.16 ng/ml), SAC (0.43 +/- 0.42 ng/ml), or LPS (0.19 +/- 0.14 ng/ml) stimulation. With adult purified T cells, high levels of IL-10 and IL-4 were measured following CD3 plus CD28 stimulation, and the amounts of both T-helper type-2 (Th2) cytokines decreased following the addition of phorbol myristate acetate (PMA), whereas the synthesis of the Th1 cytokines IL-2 and IFN-gamma was enhanced. When PBMC were stimulated with a CD3 mAb and different other cytokines were added, strong enhancement of IL-10 production was seen upon the addition of IL-2, IL-4, IL-7, IL-12 and IFN-gamma, whereas inhibition was found with transforming growth factor-beta 1 (TGF-beta 1). These data illustrate that in freshly isolated PBMC large amounts of IL-10 can be induced rapidly by appropriate mAb stimulation, and that even in freshly isolated cells IL-4 and IL-10 show signs of parallel regulation.     

Blocking CD40 - CD154 and CD80/CD86 - CD28 interactions during primary allogeneic stimulation results in T cell anergy and high IL-10 production.

Although CD28 triggering provides an important co-stimulatory signal to T cells, blocking the CD80/CD86 - CD28 interaction with CTLA-4lg fusion protein is not sufficient for tolerance induction in vivo or in vitro. According to more recent data, interruption of the CD40 - CD154 interaction might complement the effect of CTLA-4lg and induce graft acceptance. We studied the effects of a blocking anti-CD40 monoclonal antibody (mAb) and/or blocking anti-CD80/anti-CD86 mAb in cultures of human peripheral blood mononuclear cells (PBMC) stimulated with allogeneic PBMC. T cells activated by alloantigens in the presence of anti-CD80, anti-CD86 and anti-CD40 entered a state of alloantigen-specific non-responsiveness as evidenced upon restimulation by lack of proliferation, cytotoxic activity, and IL-2, IL-5 and IL-13 production. IFN-gamma production during restimulation was less than in the control cultures, while the production of IL-10 was enhanced. Addition of recombinant IL-2 during the restimulation rescued alloantigen-specific activity. We conclude that the simultaneous blocking of the CD40 - CD154 and CD80/CD86 - CD28 interaction during allogeneic T cell activation induces T cell anergy. Since anergic cells induced by this treatment still produce high levels of IL-10, the latter could contribute to modulation of antigen-presenting cell activity and to bystander suppression of residual reactive T cells.     

Evidence for intact CD28 signaling in T cell hyporesponsiveness induced by the HIV-1 nef gene

Infection by human immunodeficiency virus (HIV)-1 is associated with quantitative and qualitative T cell alterations that severely impair the host's immune defense system. The molecular basis for this immunosuppression remains unclear. Peripheral blood mononuclear cells (PBMC) isolated from patients show markedly decreased interleukin (IL)-2 secretion but unaffected or even increased T helper (Th)2 cytokine production. T cell functional defects were recently reported to correlate more with T cell receptor (TcR) signaling, whereas signals provided by ligation of co-receptors CD27 and CD28 appeared to be preserved. Among the various mechanisms proposed to be involved in HIV-1-induced T cell dysfunction, we and others have reported that the nef gene product exhibited significant immunosuppressive activity. By using an inducible stably integrated nef gene, we demonstrated that Nef specifically down-regulated IL-2 and interferon (IFN)-γ produced upon TcR triggering. Here, using the same experimental system, we extended our initial observations to additional mitogenic signals, and investigated the co-stimulatory function of CD28. Nef down-regulated IL-2, but not IL-4 produced upon induction by combinations of mitogens that mimicked TcR signals together with CD28 mAb or CD28's natural ligand (CD80 and CD86). However, the co-signals provided by CD28 to up-regulate IL-2 induction were unaffected by Nef, since IL-2 produced by nef-transfected cells was proportionally enhanced to the same extent as that of control cells, either upon stimulation by the CD28 mAb or CD80 and CD86. In addition, phosphatidylinositol-3 kinase recruitement induced upon CD28 triggering was also found to be unaltered by nef expression. Together with the observation that similar levels of the Nef protein were detected in nef-transfected cells and upon infection of PBMC, these data suggest a selective immunosuppression induced by nef in human T cells by altering TcR signaling without detectable impact on CD28 co-receptor function. These data agree with the T cell defects observed in PBMC isolated from HIV-infected individuals.     

CD44 plays a co-stimulatory role in murine T cell activation: ligation of CD44 selectively co-stimulates IL-2 production, but not proliferation in TCR-stimulated murine Th1 cells

The murine CD44 receptor family is thought to be involved ina variety of lymphocyte functions, including lymphopoesis, lymphocytehoming and cell migration. Herein, we show that murine CD44also plays a role as a co-stimulatory molecule for the activationof CD4⁺ T cells. Ligation of CD44 by mAb enhanced IL-2 productionof long-term cultured, anti-CD3-stimulated T_h1 cell lines. Moreover,anti-CD44 mAb synergized with anti-CD28 mAb in exerting thiseffect. A synergism of anti-CD28 and anti-CD44 mAb to co-stimulateIL-2 production was also observed in anti-CD3- triggered, freshlyisolated splenic CD4⁺ T cells. Blocking experiments with cyclosporinA indicated that the intracellular pathways used by the CD28and CD44 molecules appear to be different. In contrast to theeffects on the IL-2 production of T_h1 cells, neither anti-CD44mAb alone nor the combination of anti-CD44 with anti-CD28 wereable to induce proliferation of anti-CD3-triggered T_h1 cells.In accordance, triggering of CD44 and/or CD28 by mAb was notsufficient to reverse the previously described ‘proliferativeblock’. This term describes the unresponsiveness of T_h1cells against IL-2, which occurs when T_h1 cells are triggeredby anti-CD3 in the absence of co-signals. These data lead usto propose a model of T_h1 cell activation which includes twofunctionally different types of co-signals: one for IL-2 productionand a separate one for proliferation.     

Activation of peripheral CD8+ T lymphocytes via CD28 plus CD2: Evidence for IL-2 gene transcription mediated by CD28 activation

It is well established that peripheral CD8+ and CD4+ T cells display different requirements for in vitro activation by mitogenic mAb. Most CD4+ T cells can be activated by anti-CD3 or mitogenic combinations of anti-CD2. In contrast, CD8+ T cells display minimal responses to CD3 activation, and no proliferation is observed via CD2 activation. Purified peripheral blood CD8+ T cells, stringently depleted of APC, have been studied for their capacity to respond to mAb directed against CD3, CD2 and CD28, used alone or in combination. It is demonstrated that proliferation can be induced by co-stimulation of CD2 and CD28. This does not require autologous APC. CD8+ T cells can also be activated by the combination of anti-CD3 plus anti-CD28 in the presence of APC, but only minimal cell proliferation is obtained in the absence of APC. The response via CD2 plus CD28 is IL-2-dependent, as demonstrated by the ability of mAb against the IL-2 receptor to block proliferation, and is almost completely inhibited by cyclosporine A (CsA). These results suggest that the signal generated by stimulation of CD28 in combination with CD2 differs from that seen with CD28 activation combined with either PMA or CD3. Induction of IL-2 gene activation in CD8+, CD28+ peripheral T cells may therefore require additional "second signals", which are not necessary for activation of CD4+ cells. One such signal might be the interaction between CD28 and its natural ligand.     

Triggering CD101 molecule on human cutaneous dendritic cells inhibits T cell proliferation via IL-10 production

Since the CD101 molecule is expressed on a major subpopulation of HLA-DR(+), CD1a(+), CD1c(+) cutaneous dendritic cells (DC), we studied the functional role of CD101 on cutaneous DC. Anti-CD101 monoclonal antibody (mAb) inhibited the proliferation of T cells induced by cutaneous DC. There was a synergistic inhibition between anti-CD101 mAb and anti-CD86/anti-CD80 mAb. Anti-CD101 mAb exerted its inhibitory effect when binding to the CD101 expressed on cutaneous DC. No positive role of CD101 putative ligand expressed by T cells in T cell proliferation was demonstrated, as T cells proliferated in response to soluble anti-CD3 mAb in the presence of CD86-transfected cells but not in the presence of CD101-transfected cells. Of major significance is the fact that IL-10 was produced by cutaneous DC after CD101 triggering with anti-CD101 mAb, while IL-10 secretion was up-regulated in mixed cutaneous DC-T cell cultures after CD101 triggering. Furthermore, IL-10-neutralizing mAb could reverse the inhibition induced by anti-CD101 mAb. Our results demonstrate that the CD101 triggering on cutaneous DC inhibits T cell proliferation via IL-10 production, suggesting an important regulatory role played by the CD101 molecule on DC during T cell activation.     

Altered Signaling Lymphocytic Activation Molecule (SLAM) Expression in HIV Infection and Redirection of HIV-Specific Responses via SLAM Triggering

Signaling lymphocytic activation molecule (SLAM) is a transmembrane lymphocytic receptor which gets rapidly upregulated following cell activation. SLAM engagement augments T cell expansion and interferon-gamma (IFN-gamma) production independently of CD28. SLAM signaling is regulated by the SLAM-associated protein. We evaluated the expression and function of SLAM on CD4(+) and CD8(+) lymphocytes in HIV-infected individuals with either recently acquired infection (Group A) or asymptomatic HIV infection (Group B) and in healthy controls (HC). Soluble antigen (HIV env peptides and tetanus toxoid)- and mitogen-stimulated proliferation and IFN-gamma and IL-10 production upon SLAM costimulation were also measured. Results showed that: (1) SLAM-expressing CD4(+) and CD8(+) lymphocytes diminish in group A patients compared to both group B patients and HC; (2) SLAM expression on CD4(+) lymphocytes is preferentially associated with the lack of CD7 on cell surface (CD4(+)CD7(-) produce IL-10 but not IFN-gamma); (3) SLAM engagement increases HIV env peptide-stimulated, but neither tetanus toxoid- nor PHA-stimulated proliferation of peripheral blood mononuclear cells (PBMC) in patients but not in HC; and (4) SLAM engagement augments IFN-gamma and reduces IL-10 production by env peptide-stimulated PBMC of HIV-infected individuals. These results demonstrate that early HIV infection results in an altered SLAM expression which correlates with a time-limited impairment of cell-mediated immunity. Furthermore, they show that triggering via SLAM potentiates HIV-specific proliferative responses with simultaneous downregulation of IL-10 and redirection of the response to TH0/TH1.     

Cyclosporin A inhibition of macrophage colony-stimulating factor (M-CSF) production by activated human T lymphocytes

M-CSF is a pleiotropic cytokine involved in the survival, proliferation, and differentiation of cells of the monocyte/macrophage lineage. M-CSF is produced by numerous cells including CD3-activated T cells. M-CSF serum levels are increased during acute graft rejection. We tested the in vitro production of M-CSF, GM-CSF, IL-2, and IL-4 by T-cell clones costimulated by CD3 and accessory activation pathways and the effects of cyclosporin A and methylprednisolone. The nine clones studied and CD4+ cells purified from peripheral blood mononuclear cells (PBMC) spontaneously produced low levels of M-CSF, which PMA and CD3 mAb strongly enhanced. In contrast to IL-2, CD28 mAb did not further enhance this production. CsA inhibited M-CSF production by clones and purified CD4 T cells. Addition of IL-2, anti IL-2, or anti CD25 mAb to the cultures demonstrated that CsA down-regulated M-CSF synthesis by activated T cells through its inhibition of IL-2 synthesis. These results could help to better understand the complex mechanisms of acute graft rejection and immunosuppression.     

Signaling via CD28 costimulates lymphokine production,but does not reverse unresponsiveness to interleukin-2 in anti-CD3 triggered Th1 cells

Previously, it has been described that the ability of murine T_h1 cells to proliferate in response to exogenous interleukin (IL)-2 is blocked when these cells are exposed to immobilized anti-CD3 antibodies. In the present study we examined whether simultaneous triggering of the T cell antigen CD28 can prevent the induction of unresponsiveness to IL-2 in T_h1 cells. We report that costimulation of T_h1 cells with anti-CD28 monoclonal antibodies (mAb) did not overcome unresponsiveness to IL-2 induced by various amounts of immobilized anti-CD3 antibodies. However, stimulation with anti-CD28 mAb strongly augmented IL-2 and interferon-γ production in anti-CD3-exposed T_h1 cells. Thus, despite the fact that anti-CD28 mAb is a potent costimulus for lymphokine production, signaling through CD28 does not seem to be sufficient to trigger proliferation in T_h1 cells activated via the T cell receptor. These data suggest the existence of at least three signals to trigger T_h1 cell activation. The first is mediated by ligation of the T cell receptor. One cosignal, delivered by the CD28 molecule, leads to IL-2 production. A third, still undefined, signal is required for proliferation in response to IL-2.     

Cord blood CD4+ CD45RA+ T cells achieve a lower magnitude of activation when compared with their adult counterparts.

Highly purified CD4+ CD45RA+ cells from cord blood and peripheral blood from healthy adults were studied. The levels of expression of the CD2, CD3, CD4 and CD28 antigens were similar; however, CD45 and CD45RA antigen expression were slightly lower in cord cells. The reduced expression of the CD45RA antigen on cord CD4+ T cells was confirmed in whole blood. Functional assessment revealed deficiencies in cord CD4+ CD45RA+ T cells. Interleukin-2 (IL-2) production in response to specific triggering via CD2 monoclonal antibody (mAb) alone, or CD2 mAb in combination with CD28 mAb showed marked underproduction (about 10% of adult production). When CD25 expression was examined, it was observed that the proportion of activated CD4+ CD45RA+ T cells in cord blood was lower than in adult (about 20% of adult expression). Proliferation to CD2 mAbs or CD2 + 28 mAbs of cord blood native cells was similarly depressed. Investigation of IL-2 mRNA expression under these stimulatory conditions paralleled the results observed for CD25 expression, IL-2 production and proliferation. When phorbol 12-myristate 13-acetate (PMA) was added to the cells triggered with CD2 + 28mAbs, the responses examined were enhanced in both cord and adult blood with no significant differences between the groups. These findings suggest that under identical conditions of stimulation, purified cord blood CD4+ CD45RA+ T cells do not acquire similar activation status as their adult counterparts. These findings may help in understanding the reduced graft-versus-host disease (GVHD) observed in cord blood stem cell transplantation.     

Engagement of IL-1 receptor accessory protein (IL-1RAcP) with the monoclonal antibody AY19 provides co-activating signals and prolongs the CD2-induced proliferation of peripheral blood lymphocytes

IL-1 receptor accessory protein (IL-1RAcP) is the second subunit required to form a functional receptor complex for IL-1α and β, IL-1F6, IL-1F8, IL1-F9 and IL-33. While it does not directly interact with the cytokines, IL-1RAcP is necessary to mediate signal transduction. We previously reported a monoclonal antibody with an unknown specificity, termed AY19, that was capable to induce a significant increase in the size of CFU-GM colonies when added to cultures of human cord blood CD34(+) hematopoietic progenitors. Here we demonstrate that AY19 mAb recognizes IL1-RAcP. We show that this adaptor molecule is significantly present on peripheral blood monocytes and lymphocytes including CD4(+) and CD8(+) T lymphocytes, B and NK cells. Interestingly, its expression is found increased on CD127(low)CD4(+)CD25(high) T cells when compared to CD127(low)CD4(+)CD25(-) T cell subset, suggesting that the level of IL-1RAcP membrane expression could allow to distinguish within CD127(low)CD4(+) T lymphocytes the CD25(high) T regulatory subset from conventional CD25(-) T lymphocytes. Functional studies reveal that addition of AY19 mAb enhances the proliferation of peripheral blood mononuclear cells (PBMC) obtained with mitogenic concentrations of PMA. Interestingly, we found that although AY19 mAb does not increase the optimal PBMC proliferation induced by a mitogenic pair of anti-CD2 mAbs it prolongs their time of proliferation. Thus, these results indicate that the anti-IL-1RAcP mAb AY19 exhibits unique functional properties by triggering co-stimulatory signals in lymphocytes.     

Anti-CD28 antibody- and IL-4-induced human T cell proliferation is sensitive to rapamycin.

Rapamycin (RAPA) is a potent immunosuppressant. In this study we investigated the effect of RAPA on T cell proliferation triggered by various stimuli in an in vitro human model. The proliferation of T cells stimulated via an alternative pathway using phorbol myristate acetate (PMA) and anti-CD28 antibody (alpha CD28) in the absence of antigen-presenting cells (APC) was strongly inhibited by RAPA. T cell proliferation provoked via a combination of CD3/TCR and CD28 pathways using anti-CD3 antibody (alpha CD3) plus alpha CD28 was also inhibited by RAPA in the presence of APC. The mitogen (phytohaemagglutinin (PHA) or alpha CD3)-induced up-regulation of expression of the IL-2 receptor alpha chain (IL-2R alpha) and the IL-4 receptor (IL-4R) was sensitive to RAPA. This suggests that RAPA's interference with the IL-2 and IL-4 autocrine loops during T cell activation might contribute to RAPA's overall immunosuppressive effect. We have further demonstrated in a two-stage culture system that RAPA strongly inhibited IL-4-stimulated proliferation of T cells, the latter being either pretreated with alpha CD3 in the presence of APC, or with PMA plus alpha CD28 in the absence of APC. The result suggests that the Ca++ influx during the pretreatment is not obligatory for T cells to achieve IL-4 responsiveness. The results also indicate that RAPA's antiproliferative effect on IL-4-stimulated T cells is not contingent on the various mechanisms of cell priming. Therefore, RAPA's major target is probably at the second stage after the priming. Our study has extended current knowledge about the effect of RAPA on human T cells.     

The capacity of the natural ligands for CD28 to drive IL-4 expression in naïve and antigen-primed CD4+ and CD8+ T cells

The B7/CD28 costimulatory pathway plays a critical role in T cell activation including Th1/Th2 differentiation. However, little is known about whether CD28 costimulation favors polarization of either Th1 and Th2 or both. Here, we show a critical role of the natural ligands for CD28 molecules (B7.2-Ig or B7.1-Ig fusion proteins), particularly in the induction of type 2 T cell polarization. Upon TCR-triggering with suboptimal doses of anti-CD3, costimulation of na?ve CD4+ T cells with anti-CD28 mAb or B7-Ig fusion proteins led to comparable levels of IFN-gamma production. Na?ve T cells could produce IL-4 when CD28 costimulation was done with B7-Ig, but not with anti-CD28. IL-4-selective upregulation was also observed when T cells from anti-OVA TCR transgenic mice were stimulated with OVA in the presence of B7-Ig. Correlating with IL-4 expression, GATA-3 expression was induced much more potently by costimulation with B7-Ig than with anti-CD28 mAb, while T-bet induction by these two costimulatory reagents was comparable. This B7 effect was also applied for na?ve and antigen-primed CD8+ T cells: IL-4-expressing CD8+ T cells were generated when na?ve and alloantigen-primed T cells were stimulated with anti-CD3 and recall antigens, respectively, in the presence of B7-Ig costimulation. Importantly, such CD8+ T cell differentiation required the coexistence of CD4+ T cells during the initial TCR stimulation. These observations indicate that both type 2 CD4 and CD8 T cell polarizations are efficiently induced via costimulation of CD28 with its natural ligands, although the differentiation of CD8+ T cells is dependent on CD4+ cells.     

IL-10 directly acts on T cells by specifically altering the CD28 co-stimulation pathway

IL-10 induces T cell anergy in numerous mouse models and specific immunotherapy of allergy in humans. Here, we demonstrate that IL-10 directly acts on T cells which are stimulated via CD28 by efficiently blocking proliferation and cytokine production. T cells tolerized by IL-10 showed high viability and the unresponsive state was reversed by anti-CD3 monoclonal antibody (mAb) stimulation and IL-2, but not by anti-CD28 mAb stimulation. Signal transduction via CD28 requires CD28 tyrosine phosphorylation and binding of phosphatidylinositol 3-kinase. IL-10 inhibited tyrosine phosphorylation of CD28; thus, the phosphatidylinositol 3-kinase binding to CD28 was blocked. Consequently, IL-10 inhibited the antigen-induced secretion of both Th1 and Th2 cytokines, including IL-2, IFN-gamma, IL-4, IL-5 and IL-13. Furthermore, neutralization of endogenously produced IL-10 significantly increased T cell proliferation and both Th1 and Th2 cytokine production in vitro. Using superantigen stimuli, T cell suppression by IL-10 was merely induced at low doses when co-stimulation by CD28 was essential. Together, these data demonstrate that IL-10 directly acts on the CD28 signaling pathway and this represents an important T cell suppression mechanism leading to anergy.     

Functional dichotomy of plasmacytoid dendritic cells: antigen-specific activation of T cells versus production of type I interferon

Human plasmacytoid dendritic cells (PDC) are believed to link innate and adaptive immunity by producing type I interferon (IFN-I) and triggering adaptive T cell-mediated immunity. However, it remains elusive to which degree both PDC functions are linked. Here we show that CMV antigen targeted to PDC using a CD303 (blood dendritic cell antigen 2, BDCA-2) mAb is rapidly endocytosed and traffics via early sorting endosomes to emerging MHC-enriched compartments. Both processes occur independently of TLR ligand stimulation. Restimulation of CMV-specific CD4(+) effector-memory T helper cells by autologous PDC and induction of IFN-I production in PDC are dependent on appropriate stimulation. Type B CpG oligonucleotide (CpG-B)-stimulated PDC efficiently process and present CMV antigen and are thus capable of stimulating CMV-specific effector-memory T helper cells. CpG-A-stimulated PDC produce large amounts of IFN-I and express programmed death receptor-1 ligand 1. CpG-A plus CpG-B-co-stimulated PDC behave like CpG-B-stimulated PDC, suggesting that antigen processing and presentation in PDC is dependent on stimulation that concurrently inhibits IFN-I production. In vivo targeting of antigens to PDC via CD303 combined with appropriate PDC stimulation may allow induction of specific T cell activation.     

Massive production of Th2 cytokines by human CD4+ effector T cells transiently expressing the natural killer cell marker CD57/HNK1.

We have reported previously that uncommitted human CD4+ CD45RO- T cells default to the T-helper type 1 (Th1) pathway, if they are costimulated by anti-CD3 plus anti-CD28 monoclonal antibodies (mAb). In contrast, 5% of the uncommitted T cells differentiate into Th2 cells, if they are stimulated by anti-CD28 plus interleukin-2 (IL-2) in the absence of T-cell receptor (TCR) signals. The anti-CD28/IL-2-induced proliferation (and the resulting Th2 commitment) was not affected by neutralizing anti-IL-4 mAb, suggesting a non-conventional IL-4-independent Th2 differentiation pathway. Here we report that the respective CD4+ Th2 cells (but not the Th1 cells) coexpressed the natural killer (NK) cell marker HNK1/CD57. Expression of CD57 on Th2 cells required CD28 stimulation, and was suppressed by CD3/TCR signals. However, Th2 effector cells displayed a TCR V beta-chain usage comparable to that of committed Th1 cells (with V beta 8 dominating). Our data suggest that expression of CD57 on human CD4 T cells may be associated with defined stages of Th2 cell activation/differentiation, and may not necessarily characterize a separate T-cell lineage. The induction of cytokine production and B-cell helper function in both Th1 and Th2 populations required CD3/TCR signalling in costimulation with anti-CD28 or IL-2. Importantly, anti-CD28/IL-2-primed Th2 cells readily secreted IL-4 and induced IgE production by surface IgE- B cells in response to the first TCR signal and independent of previous contact with IL-4. Therefore, CD4+ CD57+ T cells responded comparably to murine CD4+ NK1.1+ T cells, which are critical for the development of Th2/IgE immune responses in vivo. The possible role of human CD4+ CD57/HNK1+ Th2-like cells in cancer, infection and allergy is discussed.     

Proliferative response of human CD4+ T lymphocytes stimulated by the lectin jacalin

The Galβ(1–3)GalNAc-binding lectin jacalin is known to specifically induce the proliferation of human CD4⁺ T lymphocytes in the presence of autologous monocytes and to interact with the CD4 molecule and block HIV-1 infection of CD4⁺ cells. We further show that jacalin-induced proliferation is characterized by an unusual pattern of T cell activation and cytokine production by human peripheral blood mononuclear cells (PBMC). A cognate interaction between T cells and monocytes was critical for jacalin-induced proliferation, and human recombinant interleukin (IL)-1 and IL-6 did not replace the co-stimulatory activity of monocytes. Blocking studies using monoclonal antibodies (mAb) point out the possible importance of two molecular pathways of interaction, the CD2/LFA-3 and LFA-1/ICAM-1 pathways. One out of two anti-CD4 mAb abolished jacalin responsiveness. Jacalin induced interferon-γ and high IL-6 secretion, mostly by monocytes, and no detectable IL-2 synthesis or secretion by PBMC. In contrast, jacalin-stimulated Jurkat T cells secreted IL-2. CD3^? Jurkat cell variants failed to secrete IL-2, suggesting the involvement of the T cell receptor/CD3 complex pathway in jacalin signaling. IL-2 secretion by CD4^? Jurkat variant cells was delayed and lowered. In addition to CD4, jacalin interacts with the CD5 molecule. Jacalin-CD4 interaction and the proliferation of PBMC, as well as IL-2 secretion by Jurkat cells were inhibited by specific jacalin-competitive sugars.