CD4 and signal transduction

Enolase 1 of Candida albicans binds human CD4+ T cells and modulates naïve and memory responses

To obtain a better understanding of the biology behind life-threatening fungal infections caused by Candida albicans, we recently conducted an in silico screening for fungal and host protein interaction partners. We report here that the extracellular domain of human CD4 binds to the moonlighting protein enolase 1 (Eno1) of C. albicans as predicted bioinformatically. By using different anti-CD4 monoclonal antibodies, we determined that C. albicans Eno1 (CaEno1) primarily binds to the extracellular domain 3 of CD4. Functionally, we observed that CaEno1 binding to CD4 activated lymphocyte-specific protein tyrosine kinase (LCK), which was also the case for anti-CD4 monoclonal antibodies tested in parallel. CaEno1 binding to naïve human CD4+ T cells skewed cytokine secretion toward a Th2 profile indicative of poor fungal control. Moreover, CaEno1 inhibited human memory CD4+ T-cell recall responses. Therapeutically, CD4+ T cells transduced with a p41/Crf1-specific T-cell receptor developed for adoptive T-cell therapy were not inhibited by CaEno1 in vitro. Together, the interaction of human CD4+ T cells with CaEno1 modulated host CD4+ T-cell responses in favor of the fungus. Thus, CaEno1 mediates not only immune evasion through its interference with complement regulators but also through the direct modulation of CD4+ T-cell responses.

CD4 ligation on human blood monocytes triggers macrophage differentiation and enhances HIV infection

A unique aspect of human monocytes, compared to monocytes from many other species, is that they express the CD4 molecule. However, the role of the CD4 molecule in human monocyte development and function is not known. We determined that the activation of CD4 via interaction with major histocompatibility complex class II (MHC-II) triggers cytokine expression and the differentiation of human monocytes into functional mature macrophages. Importantly, we determined that CD4 activation induces intracellular signaling in monocytes and that inhibition of the MAPK and Src family kinase pathways blocked the ability of CD4 ligation to trigger macrophage differentiation. We observed that ligation of CD4 by MHC-II on activated endothelial cells induced CD4-mediated macrophage differentiation of blood monocytes. Finally, CD4 ligation by MHC-II increases the susceptibility of blood-derived monocytes to HIV binding and subsequent infection. Altogether, our studies have identified a novel function for the CD4 molecule on peripheral monocytes and suggest that a unique set of events that lead to innate immune activation differ between humans and mice. Further, these events can have effects on HIV infection and persistence in the macrophage compartment.

IMPORTANCE

The CD4 molecule, as the primary receptor for HIV, plays an important role in HIV pathogenesis. There are many cell types that express CD4 other than the primary HIV target, the CD4(+) T cell. Other than allowing HIV infection, the role of the CD4 molecule on human monocytes or macrophages is not known. We were interested in determining the role of CD4 in human monocyte/macrophage development and function and the potential effects of this on HIV infection. We identified a role for the CD4 molecule in triggering the activation and development of a monocyte into a macrophage following its ligation. Activation of the monocyte through the CD4 molecule in this manner increases the ability of monocytes to bind to and become infected with HIV. Our studies have identified a novel function for the CD4 molecule on peripheral monocytes in triggering macrophage development that has direct consequences for HIV infection.

The CD4 molecule on CD8+ T lymphocytes directly enhances the immune response to viral and cellular antigens

CD8+ T lymphocytes play a major role in cellular-mediated immune responses to foreign antigen. We have previously demonstrated that costimulation of purified human CD8+ T cells induces de novo expression of the CD4 molecule and that ligation of CD4 on this cell type modulates CD8+ T cell activity in vitro. Herein, we investigate how the CD4 molecule expressed on murine CD8+ T cells contributes to CD8+ cell responses in vivo by employing adoptive transfer of CD8 cells from CD4 knockout mice into severe combined immunodeficient (SCID) recipients. Transfer of these cells into syngeneic SCID mice resulted in a decreased immune response to infection by lymphocytic choriomeningitis virus. These decreased responses occurred even in the presence of CD4+ T cells, indicating that this was truly a CD8-cell defect. Similarly, transfer of CD8+ T cells incapable of expressing CD4 into allogeneic SCID mice resulted in a decreased response to alloantigens compared with that of normal CD8+ T cells. Therefore, CD4 expression on CD8 T lymphocytes modulates cytotoxic T lymphocyte function and is critical in vivo for optimal cell-mediated immunity to viral and alloantigens.

CD4 on CD8(+) T cells directly enhances effector function and is a target for HIV infection

Costimulation of purified CD8(+) T lymphocytes induces de novo expression of CD4, suggesting a previously unrecognized function for this molecule in the immune response. Here, we report that the CD4 molecule plays a direct role in CD8(+) T cell function by modulating expression of IFN-gamma and Fas ligand, two important CD8(+) T cell effector molecules. CD4 expression also allows infection of CD8 cells by HIV, which results in down-regulation of the CD4 molecule and impairs the induction of IFN-gamma, Fas ligand, and the cytotoxic responses of activated CD8(+) T cells. Thus, the CD4 molecule plays a direct role in CD8 T cell function, and infection of these cells by HIV provides an additional reservoir for the virus and also may contribute to the immunodeficiency seen in HIV disease.

Irreversible loss of donor blood leucocyte activation may explain a paucity of transfusion-associated graft-versus-host disease from stored blood

Transfusion-associated graft-versus-host disease (TA-GVHD) is usually a fatal outcome of blood transfusion therapy, caused by viable leucocytes contained in the donor blood. Most cases of TA-GVHD occur when less than 4-d-old blood is transfused. We therefore examined the molecular changes that occur during storage that may account for the paucity of TA-GVHD following infusion of older blood. Leucocyte number and viability were essentially unchanged from freshly obtained blood, but the expression of cell-surface lymphocyte activation antigens (CD3, CD4, CD28, CD2, CD45) decreased rapidly within the first 24 h and continued to fall to less than 20% of original levels by d 9 of storage at 4 degrees C. The decrease in CD antigen expression directly correlated with a decreasing ability to induce activation of the T-lymphocyte cellular signal transduction pathway. As a result, cells became less responsive in a mixed lymphocyte culture (MLC) by d 3, with abrogation of the MLC responsiveness by d 5. Donor leucocytes stored for 4 d or less at 4 degrees C were able to partially re-express CD antigens and reconstitute their signalling pathway when placed at 37 degrees C. whereas those stored for more than 4 d were not. These irreversible changes result from a permanent downregulation of donor cell protein synthesis. These findings provide a mechanism to explain the paucity of TA-GVHD following transfusion of blood that is more than 4 d-old. Further study may show that aged blood provides additional assurances for the prevention of TA-GVHD; however, use of aged blood should not replace current protocols using irradiation.

HIV-1 gp120 and chemokines activate ion channels in primary macrophages through CCR5 and CXCR4 stimulation

HIV type 1 (HIV-1) uses the chemokine receptors CCR5 and CXCR4 as coreceptors for entry into target cells. Here we show that the HIV-1 envelope gp120 (Env) activates multiple ionic signaling responses in primary human macrophages, which are important targets for HIV-1 in vivo. Env from both CCR5-dependent JRFL (R5) and CXCR4-dependent IIIB (X4) HIV-1 opened calcium-activated potassium (K(Ca)), chloride, and calcium-permeant nonselective cation channels in macrophages. These signals were mediated by CCR5 and CXCR4 because macrophages lacking CCR5 failed to respond to JRFL and an inhibitor of CXCR4 blocked ion current activation by IIIB. MIP-1beta and SDF-1alpha, chemokine ligands for CCR5 and CXCR4, respectively, also activated K(Ca) and Cl(-) currents in macrophages, but nonselective cation channel activation was unique to gp120. Intracellular Ca(2+) levels were also elevated by gp120. The patterns of activation mediated by CCR5 and CXCR4 were qualitatively similar but quantitatively distinct, as R5 Env activated the K(Ca) current more frequently, elicited Cl(-) currents that were approximately 2-fold greater in amplitude, and elevated intracellular Ca(+2) to higher peak and steady-state levels. Env from R5 and X4 primary isolates evoked similar current responses as the corresponding prototype strains. Thus, the interaction of HIV-1 gp120 with CCR5 or CXCR4 evokes complex and distinct signaling responses in primary macrophages, and gp120-evoked signals differ from those activated by the coreceptors' chemokine ligands. Intracellular signaling responses of macrophages to HIV-1 may modulate postentry steps of infection and cell functions apart from infection.

Inhibition of contact sensitivity in human CD4+ transgenic mice by human CD4-specific monoclonal antibodies: CD4+ T-cell depletion is not required

Clenoliximab and keliximab are monkey/human chimeric monoclonal antibodies (mAbs) of the immunoglobulin G4 (IgG4) and IgG1 isotypes, respectively, that recognize the same epitope on human CD4. The two mAbs possess identical idiotypes and exhibit equal affinities for CD4. Upon administration of these mAbs to mice that express a human CD4 transgene, but not mouse CD4 (HuCD4/Tg mice), clenoliximab and keliximab exhibited similar kinetics of binding to CD4, and induced the same degree of CD4 modulation from the cell surface, although only keliximab mediated CD4+ T-cell depletion. Epicutaneous sensitization and challenge of HuCD4/Tg mice with the hapten oxazolone resulted in a contact sensitivity response characterized by tissue swelling, and the presence of interferon-gamma (IFN-gamma) and interleukin-4 (IL-4) in the local tissue. Administration of a single 2-mg dose of either clenoliximab or keliximab to HuCD4/Tg mice prior to sensitization significantly reduced post-challenge tissue swelling, and levels of IFN-gamma and IL-4, indicating that CD4+ T-cell depletion is not required for anti-CD4 mAb-mediated inhibition of contact sensitivity. Administration of either mAb prior to challenge failed to inhibit the contact sensitivity response, indicating differential sensitivity of the afferent and efferent phases of the response to inhibition by CD4-specific mAbs. Collectively, these data indicate that CD4 functions as a positive regulatory molecule in the contact sensitivity response.

Inhibition of Tyrosine Kinase Activation Blocks the Down-Regulation of CXC Chemokine Receptor 4 by HIV-1 gp120 in CD4+ T Cells

Because the binding of HIV-1 envelope to CD4 initiates a configurational change in glycoprotein 120 (gp120), enabling it to interact with fusion coreceptors, we investigated how this process interferes with the expression and function of CXC chemokine receptor 4 (CXCR4) in CD4+ T lymphocytes. A recombinant gp120 (MN), after preincubation with CD4+ T lymphocytes, significantly inhibited the binding and chemotaxis of the cells in response to the CXCR4 ligand stromal cell-derived factor-1α (SDF-1α), accompanied by a markedly reduced surface expression of CXCR4. gp120, but not SDF-1α, induced rapid tyrosine phosphorylation of src-like kinase p56lck in CD4+ T cells, whereas both gp120 and SDF-1α caused phosphorylation of the CXCR4. The tyrosine kinase inhibitor herbimycin A abolished the phosphorylation of p56lck and CXCR4 induced by gp120 in association with maintenance of normal expression of cell surface CXCR4 and a migratory response to SDF-1α. Thus, a CD4-associated signaling molecule(s) including p56lck is activated by gp120 and is required for the down-regulation of CXCR4.

Requirement for Efficient Interactions Between CD4 and MHC Class II Molecules for Survival of Resting CD4+ T Lymphocytes In Vivo and for Activation-Induced Cell Death

Regulation of homeostasis in the immune system includes mechanisms that promote survival of resting T lymphocytes, and others that control activation-induced cell death (AICD). In this study, we report on the use of a transgenic mouse model to test the role of CD4-MHC class II interactions for the susceptibility of CD4+ T lymphocytes to AICD, and for the survival of resting CD4+ T cells in peripheral lymphoid organs. The only I-Aβ gene expressed in these mice is an Aβk transgene with a mutation that prevents MHC class II molecules from interacting with CD4. We show increased apoptosis in CD4+ T lymphocytes derived from wild-type, but not from mutant Aβk transgenic mice following stimulation with staphylococcal enterotoxin A. Therefore, AICD may be impaired in CD4+ T cells derived from mutant Aβk transgenic mice. Importantly, we observed much higher apoptosis in resting CD4+ T cells from mutant Aβk transgenic mice than from wild-type mice. Furthermore, resting CD4+ T cells from mutant Aβk transgenic mice expressed higher levels of cell surface CD95 (Fas, APO-1). Ab-mediated cross-linking of CD95 further increased apoptosis in CD4+ T cells from mutant Aβk transgenic mice, but not from wild-type mice, suggesting apoptosis involved CD95 signaling. When cocultured with APC-expressing wild-type MHC class II molecules, apoptosis in resting CD4+ T lymphocytes from mutant Aβk transgenic mice was reduced. Our results show for the first time that interactions between CD4 and MHC class II molecules are required for the survival of resting CD4+ T cells in peripheral lymphoid organs.

Selection and Function of CD4+ T Lymphocytes in Transgenic Mice Expressing Mutant MHC Class II Molecules Deficient in Their Interaction with CD4

Interactions of the T cell coreceptors, CD4 and CD8, with MHC molecules participate in regulating thymocyte development and T lymphocyte activation and differentiation to memory T cells. However, the exact roles of these interactions in normal T cell development and function remain unclear. CD4 interacts with class II MHC7 molecules via several noncontiguous regions in both the class II MHC α- and β-chains. We have introduced a double mutation that disrupts interaction with CD4 into the I-Aβk gene and used this construct to generate transgenic mice expressing only mutant class II MHC. Although CD4+ thymocytes matured to the single-positive stage in these mice, their frequency was reduced by threefold compared with that of wild-type transgenics. Positive selection of CD4+ T cells in the mutant transgenic mice may have been mediated by TCRs with a higher than usual affinity for class II MHC/Ag complexes. In Aβk mutant transgenics, peripheral CD4+ lymphocytes promoted B cell differentiation to plasma cells. These CD4+ T cells also secreted IFN-γ in response to various stimuli (e.g., protein Ag, bacterial superantigen, and alloantigen), but were deficient in IL-2 secretion. Interactions between CD4 and class II MHC molecules appeared to regulate lymphokine production, with a strong bias toward IFN-γ and against IL-2 in the absence of these interactions. Our results have implications for the manipulation of T cell-dependent immune responses.

HIV-1 Envelope gp120 Inhibits the Monocyte Response to Chemokines Through CD4 Signal-Dependent Chemokine Receptor Down-Regulation

Since HIV-1 infection results in severe immunosuppression, and the envelope protein gp120 has been reported to interact with some of the chemokine receptors on human T lymphocytes, we postulated that gp120 may also affect monocyte activation by a variety of chemokines. This study shows that human peripheral blood monocytes when preincubated with gp120 either purified from laboratory-adapted strains or as recombinant proteins exhibited markedly reduced binding, calcium mobilization, and chemotactic response to chemokines. The gp-120-pretreated monocytes also showed a decreased response to FMLP. This broad inhibition of monocyte activation by chemoattractants required interaction of gp120 with CD4, since the effect of gp120 was only observed in CD4+ monocytes and in HEK 293 cells only if cotransfected with both chemokine receptors and an intact CD4, but not a CD4 lacking its cytoplasmic domain. Anti-CD4 mAbs mimicked the effect of gp120, and both anti-CD4 Ab and gp120 caused internalization of CXCR4 in HEK 293 cells provided they also expressed CD4. Staurosporine blocked the inhibitory effect of gp120 on monocytes, suggesting that cellular signaling was required for gp120 to inhibit the response of CD4+ cells to chemoattractants. Our study demonstrates a broad suppressive effect of gp120 on monocyte activation by chemoattractants through the down-regulation of cell surface receptors. Thus, gp120 may be used by HIV-1 to disarm the monocyte response to inflammatory stimulation.

HIV-1 envelope gp41 is a potent inhibitor of chemoattractant receptor expression and function in monocytes

HIV-1 uses CD4 and chemokine receptors as cofactors for cellular entry. The viral envelope transmembrane protein gp41 is thought to participate in viral fusion with CD4(+) cells. We investigated whether gp41 interacts with chemokine receptors on human monocytes by testing its effect on the capacity of cells to respond to chemokine stimulation. Monocytes preincubated with gp41 of the MN strain showed markedly reduced binding, calcium mobilization, and chemotaxis in response to a variety of chemokines as well as to the bacterial peptide fMLP. This generalized inhibition of monocyte activation by chemoattractants required the presence of CD4, since the effect of gp41 was only observed in CD4(+) monocytes and in HEK293 cells cotransfected with chemokine receptors and an intact CD4, but not a CD4 lacking its cytoplasmic domain. Confocal microscopy showed that gp41 caused internalization of CXCR4 in HEK293 cells provided they were also cotransfected with intact CD4. In addition, pretreatment of monocytes with protein kinase C inhibitors partially reversed the inhibitory effect of gp41. Thus, gp41, which had not previously been implicated as interacting with HIV-1 fusion cofactors, downregulates chemoattractant receptors on monocytes by a CD4-dependent pathway.

A novel complex of proteins binds the HIV-1 promoter upon virus interaction with CD4

HIV-1Lai13EM is a mutant isolate which is less sensitive than the parental HIV-1Lai strain to an in vitro treatment with 13B8-2 anti-CD4 monoclonal antibody (mAb) that generally inhibits transcription of HIV-1 and HIV-2. In contrast to other clade B viruses, this isolate carries a point mutation G > A at position -188 of the viral promoter. The fact that HIV-1NDK, a clade D virus insensitive to 13B8-2 mAb, also carries an A nucleotide at this position has brought our attention to the sequence surrounding position -188. Here we analyzed whether a DNA-binding molecule interacts with this region. Electrophoretic mobility shift assays performed with the -201/-175 HIV-1Lai wild-type sequence or the sequence containing a point mutation G > A at position -188 demonstrated their ability to bind a heterotrimeric complex induced in CEM cells by stimulation with heat-inactivated HIV-1.

Physical association between STAT1 and the interferon-inducible protein kinase PKR and implications for interferon and double-stranded RNA signaling pathways

The interferon-inducible double-stranded RNA protein kinase PKR controls protein synthesis through the phosphorylation of eukaryotic translation initiation factor (eIF)-2. In addition to its demonstrated role in translational control, several reports have suggested a transcriptional role for PKR. Here we report that PKR is involved in IFN- and dsRNA-signaling pathways by modulating the function of the signal transducer and activator of transcription STAT1. We also show that PKR associates with STAT1 in mouse and human cells. The association is not a kinase-substrate interaction since STAT1 phosphorylation is not modified by PKR in vitro or in vivo. In addition, the formation of the PKR-STAT1 complex is not dependent upon the enzymatic activity of PKR but does require the dsRNA-binding domain of PKR. Moreover, there is a concomitant decrease in PKR-STAT1 interaction and increase in STAT1 DNA binding in response to IFNs or dsRNA. These findings suggest that PKR plays an important role in IFN and dsRNA-signaling pathways by modulating the transcriptional function of STAT1.