Human CD38 interferes with HIV-1 fusion through a sequence homologous to the V3 loop of the viral envelope glycoprotein gp120

The calcium signaling enzyme CD38 - a paradigm for membrane topology defining distinct protein functions

CD38 is a single-pass transmembrane enzyme catalyzing the synthesis of two nucleotide second messengers, cyclic ADP-ribose (cADPR) from NAD and nicotinic acid adenine dinucleotide phosphate (NAADP) from NADP. The former mediates the mobilization of the endoplasmic Ca²⁺-stores in response to a wide range of stimuli, while NAADP targets the endo-lysosomal stores. CD38 not only possesses multiple enzymatic activities, it also exists in two opposite membrane orientations. Type III CD38 has the catalytic domain facing the cytosol and is responsible for producing cellular cADPR. The type II CD38 has an opposite orientation and is serving as a surface receptor mediating extracellular functions such as cell adhesion and lymphocyte activation. Its ecto-NADase activity also contributes to the recycling of external NAD released by apoptosis. Endocytosis can deliver surface type II CD38 to endo-lysosomes, which acidic environment favors the production of NAADP. This article reviews the rationale and evidence that have led to CD38 as a paradigm for membrane topology defining distinct functions of proteins. Also described is the recent discovery of a hitherto unknown cADPR-synthesizing enzyme, SARM1, ushering in a new frontier in cADPR-mediated Ca²⁺-signaling.

NAD+ metabolism: pathophysiologic mechanisms and therapeutic potential

Nicotinamide adenine dinucleotide (NAD+) and its metabolites function as critical regulators to maintain physiologic processes, enabling the plastic cells to adapt to environmental changes including nutrient perturbation, genotoxic factors, circadian disorder, infection, inflammation and xenobiotics. These effects are mainly achieved by the driving effect of NAD+ on metabolic pathways as enzyme cofactors transferring hydrogen in oxidation-reduction reactions. Besides, multiple NAD+-dependent enzymes are involved in physiology either by post-synthesis chemical modification of DNA, RNA and proteins, or releasing second messenger cyclic ADP-ribose (cADPR) and NAADP+. Prolonged disequilibrium of NAD+ metabolism disturbs the physiological functions, resulting in diseases including metabolic diseases, cancer, aging and neurodegeneration disorder. In this review, we summarize recent advances in our understanding of the molecular mechanisms of NAD+-regulated physiological responses to stresses, the contribution of NAD+ deficiency to various diseases via manipulating cellular communication networks and the potential new avenues for therapeutic intervention.

Roles of CD38 in the Immune Response to Infection

CD38 is a multifunctional protein widely expressed in cells from the immune system and as a soluble form in biological fluids. CD38 expression is up-regulated by an array of inflammatory mediators, and it is frequently used as a cell activation marker. Studies in animal models indicate that CD38 functional expression confers protection against infection by several bacterial and parasitic pathogens. In addition, infectious complications are associated with anti-CD38 immunotherapy. Although CD38 displays receptor and enzymatic activities that contribute to the establishment of an effective immune response, recent work raises the possibility that CD38 might also enhance the immunosuppressive potential of regulatory leukocytes. This review integrates the current knowledge on the diversity of functions mediated by CD38 in the host defense to infection.

Vitamin D treatment of peripheral blood mononuclear cells modulated immune activation and reduced susceptibility to HIV-1 infection of CD4+ T lymphocytes

INTRODUCTION

Mucosal immune activation, in the context of sexual transmission of HIV-1 infection, is crucial, as the increased presence of activated T cells enhance susceptibility to infection. In this regard, it has been proposed that immunomodulatory compounds capable of modulating immune activation, such as Vitamin D (VitD) may reduce HIV-1 transmission and might be used as a safe and cost-effective strategy for prevention. Considering this, we examined the in vitro effect of the treatment of peripheral blood mononuclear cells (PBMCs) with the active form of VitD, calcitriol, on cellular activation, function and susceptibility of CD4+ T cells to HIV-1 infection.

METHODS

We treated PBMCs from healthy HIV unexposed individuals (Co-HC) and frequently exposed, HIV-1 seronegative individuals (HESNs) from Colombia and from healthy non-exposed individuals from Canada (Ca-HC) with calcitriol and performed in vitro HIV-1 infection assays using X4- and R5-tropic HIV-1 strains respectively. In addition, we evaluated the activation and function of T cells and the expression of viral co-receptors, and select antiviral genes following calcitriol treatment.

RESULTS

Calcitriol reduced the frequency of infected CD4+ T cells and the number of viral particles per cell, for both, X4- and R5-tropic viruses tested in the Co-HC and the Ca-HC, respectively, but not in HESNs. Furthermore, in the Co-HC, calcitriol reduced the frequency of polyclonally activated T cells expressing the activation markers HLA-DR and CD38, and those HLA-DR+CD38-, whereas increased the subpopulation HLA-DR-CD38+. Calcitriol treatment also decreased production of granzyme, IL-2 and MIP-1β by T cells and increased the transcriptional expression of the inhibitor of NF-kB and the antiviral genes cathelicidin (CAMP) and APOBEC3G in PBMCs from Co-HC.

CONCLUSION

Our in vitro findings suggest that VitD treatment could reduce HIV-1 transmission through a specific modulation of the activation levels and function of T cells, and the production of antiviral factors. In conclusion, VitD remains as an interesting potential strategy to prevent HIV-1 transmission that should be further explored.

Second messenger analogues highlight unexpected substrate sensitivity of CD38: total synthesis of the hybrid "L-cyclic inosine 5'-diphosphate ribose"

The multifunctional, transmembrane glycoprotein human CD38 catalyses the synthesis of three key Ca²⁺-mobilising messengers, including cyclic adenosine 5′-diphosphate ribose (cADPR), and CD38 knockout studies have revealed the relevance of the related signalling pathways to disease. To generate inhibitors of CD38 by total synthesis, analogues based on the cyclic inosine 5′-diphosphate ribose (cIDPR) template were synthesised. In the first example of a sugar hybrid cIDPR analogue, “L-cIDPR”, the natural “northern” N1-linked D-ribose of cADPR was replaced by L-ribose. L-cIDPR is surprisingly still hydrolysed by CD38, whereas 8-Br-L-cIDPR is not cleaved, even at high enzyme concentrations. Thus, the inhibitory activity of L-cIDPR analogues appears to depend upon substitution of the base at C-8; 8-Br-L-cIDPR and 8-NH₂-L-cIDPR inhibit CD38-mediated cADPR hydrolysis (IC₅₀ 7 μM and 21 µM respectively) with 8-Br-L-cIDPR over 20-fold more potent than 8-Br-cIDPR. In contrast, L-cIDPR displays a comparative 75-fold reduction in activity, but is only ca 2-fold less potent than cIDPR itself. Molecular modelling was used to explore the interaction of the CD38 catalytic residue Glu-226 with the “northern” ribose. We propose that Glu226 still acts as the catalytic residue even for an L-sugar substrate. 8-Br-L-cIDPR potentially binds non-productively in an upside-down fashion. Results highlight the key role of the “northern” ribose in the interaction of cADPR with CD38.

Precursor Forms of Vitamin D Reduce HIV-1 Infection In Vitro

BACKGROUND

Although the anti-HIV-1 effects of vitamin D (VitD) have been reported, mechanisms behind such protection remain largely unexplored.

METHODS

The effects of two precursor forms (cholecalciferol/calciol at 0.01, 1 and 100 nM and calcidiol at 100 and 250 nM) on HIV-1 infection, immune activation, and gene expression were analyzed in vitro in cells of Colombian and Italian healthy donors. We quantified levels of released p24 by enzyme-linked immunosorbent assay, of intracellular p24 and cell-surface expression of CD38 and HLA-DR by flow cytometry, and mRNA expression of antiviral and immunoregulatory genes by real-time reverse transcription-polymerase chain reaction.

RESULTS

Cholecalciferol decreased the frequency of HIV-1-infected p24CD4 T cells and levels of p24 in supernatants in a dose-dependent manner. Moreover, the CD4CD38HLA-DR and CD4CD38HLA-DR subpopulations were more susceptible to infection but displayed the greatest cholecalciferol-induced decreases in infection rate by an X4-tropic strain. Likewise, cholecalciferol at its highest concentration decreased the frequency of CD38HLA-DR but not of CD38HLA-DR T-cell subsets. Analyzing the effects of calcidiol, the main VitD source for immune cells and an R5-tropic strain as the most frequently transmitted virus, a reduction in HIV-1 productive infection was also observed. In addition, an increase in mRNA expression of APOBEC3G and PI3 and a reduction of TRIM22 and CCR5 expression, this latter positively correlated with p24 levels, was noted.

CONCLUSIONS

VitD reduces HIV-1 infection in T cells possibly by inducing antiviral gene expression, reducing the viral co-receptor CCR5 and, at least at the highest cholecalciferol concentration, by promoting an HIV-1-restrictive CD38HLA-DR immunophenotype.

Sphingolipid Microdomains Mediate CD38 Internalization: Topography of the Endocytosis

Plasma membranes of several cell types contain specialized microdomains (or lipid rafts) enriched in sphingolipids, cholesterol, sphingomyelin, and glycosyl-phosphatidylinositol-anchored proteins. These membrane domains are characterized by detergent insolubility at low temperatures and low buoyant density. Human CD38 is the prototype of a gene family encoding surface molecules endowed with multiple functional activities. The endocytosis of the human CD38 molecule has been investigated in normal lymphocytes and in a number of leukemia- and lymphoma-derived cell lines demonstrating that internalization after CD38 ligation is a reproducible event involving only a fraction of the whole amount of the surface molecule. This study reports the results obtained by conventional, confocal, and electron microscopy on the effects induced by the engagement of the molecule with agonistic mAb, reproducing the signals mediated by its natural ligand. The results demonstrate that the endocytosis induced as consequence of CD38 ligation is preceded by a thorough rearrangement of the cell surface with formation of glycosphingolipid- and cholesterol-rich plasma membrane microdomains. These data suggest that specialized raft microdomains might be the plasma membrane structure through which CD38 translocates at intracellular level. The CD38/lipid interactions during the coated pit formation trigger a process that generate membrane curvature, considered as the first step of CD38 endocytosis. Moreover, ultrastructural studies show that early CD38+ endosomes are pleiomorphic and contain cisternal and vesicular regions. Late endosomes exhibit a complex organisation, containing uncoupled CD38-ligand multivesicular- or multilamellar-regions.

Immuno-targeting the multifunctional CD38 using nanobody

CD38, as a cell surface antigen is highly expressed in several hematologic malignancies including multiple myeloma (MM) and has been proven to be a good target for immunotherapy of the disease. CD38 is also a signaling enzyme responsible for the metabolism of two novel calcium messenger molecules. To be able to target this multifunctional protein, we generated a series of nanobodies against CD38 with high affinities. Crystal structures of the complexes of CD38 with the nanobodies were solved, identifying three separate epitopes on the carboxyl domain. Chromobodies, engineered by tagging the nanobody with fluorescence proteins, provide fast, simple and versatile tools for quantifying CD38 expression. Results confirmed that CD38 was highly expressed in malignant MM cells compared with normal white blood cells. The immunotoxin constructed by splicing the nanobody with a bacterial toxin, PE38 shows highly selective cytotoxicity against patient-derived MM cells as well as the cell lines, with half maximal effective concentration reaching as low as 10⁻¹¹ molar. The effectiveness of the immunotoxin can be further increased by stimulating CD38 expression using retinoid acid. These results set the stage for the development of clinical therapeutics as well as diagnostic screening for myeloma.

Cyclic adenosine 5'-diphosphate ribose analogs without a "southern" ribose inhibit ADP-ribosyl cyclase-hydrolase CD38

Cyclic adenosine 5'-diphosphate ribose (cADPR) analogs based on the cyclic inosine 5'-diphosphate ribose (cIDPR) template were synthesized by recently developed stereo- and regioselective N1-ribosylation. Replacing the base N9-ribose with a butyl chain generates inhibitors of cADPR hydrolysis by the human ADP-ribosyl cyclase CD38 catalytic domain (shCD38), illustrating the nonessential nature of the "southern" ribose for binding. Butyl substitution generally improves potency relative to the parent cIDPRs, and 8-amino-N9-butyl-cIDPR is comparable to the best noncovalent CD38 inhibitors to date (IC50 = 3.3 μM). Crystallographic analysis of the shCD38:8-amino-N9-butyl-cIDPR complex to a 2.05 Å resolution unexpectedly reveals an N1-hydrolyzed ligand in the active site, suggesting that it is the N6-imino form of cADPR that is hydrolyzed by CD38. While HPLC studies confirm ligand cleavage at very high protein concentrations, they indicate that hydrolysis does not occur under physiological concentrations. Taken together, these analogs confirm that the "northern" ribose is critical for CD38 activity and inhibition, provide new insight into the mechanism of cADPR hydrolysis by CD38, and may aid future inhibitor design.

'Click cyclic ADP-ribose': a neutral second messenger mimic

Analogues of the potent Ca(2+) releasing second messenger cyclic ADP-ribose (cADPR) with a 1,2,3-triazole pyrophosphate bioisostere were synthesised by click-mediated macrocyclisation. The ability to activate Ca(2+) release was surprisingly retained, and hydrolysis of cADPR by CD38 could also be inhibited, illustrating the potential of this approach to design drug-like signalling pathway modulators.

Quantitative proteomic analysis of exosomes from HIV-1-infected lymphocytic cells

HIV-1 infection causes profound effects both inside and outside of cells through multiple mechanisms, including those mediated by exosomes. Using the technique of stable isotope labeling by amino acids in cell culture, we compared protein expression patterns in the exosomal compartment of HIV-1-infected and -uninfected lymphocytic H9 cells. Of 770 proteins identified in two independent sets of exosomal samples, 14 proteins were found to be differentially expressed in the exosomal fraction of HIV-1-infected cells versus -uninfected controls. Gene Ontology survey and DAVID analysis revealed that identified proteins were enriched for functional categories such as binding. Of these 14 proteins, three immunomodulatory molecules were reproducibly identified in both replicates and included ADP-ribosyl cyclase 1 (CD38), L-lactate dehydrogenase B chain (LDHB), and Annexin A5 (ANXA5). In addition to previously reported HIV-1 associations with CD38 and LDHB, new interactions were identified and validated for ANXA5, CD38, and LDHB, which were found to bind to HIV-1 p24 and Tat. In summary, our studies reveal that exosomes released from HIV-1-infected cells are composed of a unique and quantitatively different protein signature and harbor regulatory molecules that impact the processes of cellular apoptosis (ANXA5 and LDHB) and proliferation (CD38).

Cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate (NAADP) as messengers for calcium mobilization

Cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate were discovered >2 decades ago. That they are second messengers for mobilizing Ca(2+) stores has since been firmly established. Separate stores and distinct Ca(2+) channels are targeted, with cyclic ADP-ribose acting on the ryanodine receptors in the endoplasmic reticulum, whereas nicotinic acid adenine dinucleotide phosphate mobilizes the endolysosomes via the two-pore channels. Despite the structural and functional differences, both messengers are synthesized by a ubiquitous enzyme, CD38, whose crystal structure and catalytic mechanism have now been well elucidated. How this novel signaling enzyme is regulated remains largely unknown and is the focus of this minireview.

CD38 identifies a hypo-proliferative IL-13-secreting CD4+ T-cell subset that does not fit into existing naive and memory phenotype paradigms

CD38 is commonly regarded as an activation marker for human T cells. Herein, we show that CD38 expression identifies a hypo-proliferative CD4(+) T-cell subset that, following TCR stimulation, retains expression of naive cell surface markers including CD45RA, CD62L and CCR7. Hypo-proliferation was mediated by reduced CD25 up-regulation upon TCR stimulation compared to CD4(+) CD38(-) cells and lack of responsiveness to exogenous IL-2. Instead, CD4(+) CD38(+) T cells expressed CD127, and hypo-proliferation was reversed by addition of IL-7, further associated with increased STAT5 phosphorylation. This phenotype was exacerbated by addition of an agonistic CD38-binding antibody, suggesting that signaling through CD38 promotes this cell profile. Activated CD4(+) CD38(+) cells had a bias towards IL-13 secretion, but not other Th2 cytokines such as IL-4 or IL-5. In comparison, the CD4(+) CD38(-) cells had a clear bias towards secretion of Th1-associated cytokines IFN-γ and TNF. The existence of such CD4(+) CD38(+) T cells may play an important role in pathologies such as asthma, which are associated with IL-13, but not IL-4 and IL-5. Coupled with responsiveness to IL-7 but not IL-2, and the involvement of CD38 ligation, our results highlight a unique T-cell subpopulation that does not fit into existing naive and memory cell paradigms.

CD38 increases CXCL12-mediated signals and homing of chronic lymphocytic leukemia cells

Homing of chronic lymphocytic leukemia (CLL) cells to sites favoring growth, a critical step in disease progression, is principally coordinated by the CXCL12/CXCR4 axis. A cohort of 62 CLL patients was divided into migrating and nonmigrating subsets according to chemotaxis toward CXCL12. Migrating patients phosphorylated extracellular signal-regulated kinase 1/2 (ERK1/2) proteins more than nonmigrating patients (P<0.0002). CD38 expression was the parameter most strongly associated with heightened CXCL12 signaling (P<0.0001), confirmed by independent statistical approaches. Consistent with this observation, CD38(-) CLL cells in samples with bimodal CD38 expression responded less to CXCL12 than the intact clone (P=0.003). Furthermore, lentivirus-induced de novo expression of CD38 was paralleled by increased responses to CXCL12, as compared with cells infected with a control virus. CD38 ligation with agonistic monoclonal antibodies (mAbs) enhanced CXCL12 signaling, whereas blocking anti-CD38 mAbs inhibited chemokine effects in vitro. This is attributed to physical proximity on the membrane between CD38 and CXCR4 (the CXCL12 receptor), as shown by (i) coimmunoprecipitation and (ii) confocal microscopy experiments. Blocking anti-CD38 mAbs significantly compromised homing of CLL cells from blood to lymphoid organs in a mouse model. These results indicate that CD38 synergizes with the CXCR4 pathway and support the working hypothesis that migration is a central step in disease progression.

Evaluation of the antiretroviral effects of a PEG-conjugated peptide derived from human CD38

OBJECTIVE

Cell infection by HIV-1 is inhibited by both the expression of CD38 and a soluble peptide (sCD38p) corresponding to its extracellular membrane-proximal amino acid sequence (amino acids 51 - 74). We show here the effects of PEG conjugation to sCD38p and provide new insights into the mechanisms behind the anti-HIV-1 effects of CD38 and derived peptides.

RESEARCH DESIGN/METHODS

In-vitro and in-silico study.

RESULTS

PEGylation of sCD38p increased its ability to inhibit replication of HIV-1 in MT-4 cells and syncytia formation in cocultures of MT-2 and persistently HIV-1(IIIB)-infected H9(IIIB) cells. In silico modeling suggests that sCD38p and CD4 form stable heterodimers involving, among others, an interaction between lysine 57 (K57) of CD38 and a groove in the CD4 receptor, which, in CD4/gp120 complexes, is partially occupied by a lysine residue of the HIV-1 envelope glycoprotein. K57 substitution with a glycine in sCD38p impaired its ability to inhibit syncytia formation in MT-2/H9(IIIB) cell cocultures and gp120 binding to CD4 in a mouse T cell line expressing human but not mouse CD4.

CONCLUSIONS

PEGylation significantly improves the anti-HIV-1 activity of sCD38p, whose effect is probably due to competition with gp120 for a common binding site on CD4 although other mechanisms cannot be excluded so far. The inhibitory concentrations of the sCD38p-PEG as well as its poor toxicity, merit further consideration in anti-HIV-1 strategies.

NAADP: a universal Ca2+ trigger

Cells possess multiple calcium ion (Ca2+) stores and multiple messenger molecules to mobilize them. These include d-myo-inositol 1,4,5-trisphosphate (IP(3)), cyclic adenosine diphosphoribose (cADPR), and the most recently identified Ca2+-mobilizing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP), which acts on a wide spectrum of cells, from plant cells to mammalian cells. Accumulating evidence indicates that NAADP targets both acidic (lysosome-like) Ca2+ stores and endoplasmic reticular stores. Recent studies in invertebrate and mammalian cells suggest that NAADP provides an initiating Ca2+ signal, which is amplified by cADPR- or IP(3)-dependent mechanisms (or both) through Ca2+-induced Ca2+ release. Diverse stimuli activate a rapid rise of endogenous NAADP concentration, resulting in severalfold increases of NAADP over basal values within seconds. The enzyme CD38 can catalyze both the synthesis and hydrolysis of NAADP, making it ideal for effecting the rapid metabolism of NAADP. The crystal structure of CD38 and the structures of its various substrate complexes have now been determined, clarifying the mechanism of its multifunctional catalysis. We anticipate that these advances will lead to the unmasking of all the key components of the Ca2+ signaling pathway mediated by NAADP.

Evolution and function of the ADP ribosyl cyclase/CD38 gene family in physiology and pathology

The membrane proteins CD38 and CD157 belong to an evolutionarily conserved family of enzymes that play crucial roles in human physiology. Expressed in distinct patterns in most tissues, CD38 (and CD157) cleaves NAD(+) and NADP(+), generating cyclic ADP ribose (cADPR), NAADP, and ADPR. These reaction products are essential for the regulation of intracellular Ca(2+), the most ancient and universal cell signaling system. The entire family of enzymes controls complex processes, including egg fertilization, cell activation and proliferation, muscle contraction, hormone secretion, and immune responses. Over the course of evolution, the molecules have developed the ability to interact laterally and frontally with other surface proteins and have acquired receptor-like features. As detailed in this review, the loss of CD38 function is associated with impaired immune responses, metabolic disturbances, and behavioral modifications in mice. CD38 is a powerful disease marker for human leukemias and myelomas, is directly involved in the pathogenesis and outcome of human immunodeficiency virus infection and chronic lymphocytic leukemia, and controls insulin release and the development of diabetes. Here, the data concerning diseases are examined in view of potential clinical applications in diagnosis, prognosis, and therapy. The concluding remarks try to frame all of the currently available information within a unified working model that takes into account both the enzymatic and receptorial functions of the molecules.

Regulation of intracellular levels of NAD: a novel role for CD38

Nicotinamide adenine dinucleotide (NAD) plays key roles in many cellular functions. In addition to its well-known role in energy metabolism, NAD also plays a role in signal transduction, ageing, and cellular injury. NAD is also involved in many signal transduction pathways. Therefore, it is imperative to understand the mechanisms that control intracellular NAD levels. However, to date, the mechanisms that regulate intracellular levels of NAD have not been completely elucidated. CD38 is a multifunctional enzyme ubiquitously distributed in mammalian tissues. CD38 has been implicated as the enzyme responsible for the synthesis of the second messengers. However, its major enzymatic activity is the hydrolysis of NAD, in fact, CD38 will generate one molecule of cADPR for every 100 molecules of NAD hydrolyzed. To date, the role of CD38 as a modulator of levels of NAD has not been explored. We postulated that CD38 is the major NADase in mammalian cells and that it regulates intracellular NAD levels. In the current studies we examined the NADase activities and NAD levels in a variety of tissues from both wild-type and CD38 deficient mice. In accordance with our hypothesis, we found that tissue levels of NAD in CD38 deficient mice are 10- to 20-fold higher than in wild-type animals. In addition, NADase activity in the plasma membrane, mitochondria, sarcoplasmic reticulum, and nuclei is essentially absent in most tissues from CD38 deficient mice. These data support the novel concept that CD38 is a major regulator of cellular NAD levels. These findings have implications for understanding the mechanisms that regulate intracellular NAD levels and its role in energy homeostasis, signal transduction, and ageing.

Crystal structure of human CD38 extracellular domain

Human CD38 is a multifunctional protein involved in diverse functions. As an enzyme, it is responsible for the synthesis of two Ca2+ messengers, cADPR and NAADP; as an antigen, it is involved in regulating cell adhesion, differentiation, and proliferation. Besides, CD38 is a marker of progression of HIV-1 infection and a negative prognostic marker of B-CLL. We have determined the crystal structure of the soluble extracellular domain of human CD38 to 1.9 A resolution. The enzyme's overall topology is similar to the related proteins CD157 and the Aplysia ADP-ribosyl cyclase, except with large structural changes at the two termini. The extended positively charged N terminus has lateral associations with the other CD38 molecule in the crystallographic asymmetric unit. The analysis of the CD38 substrate binding models revealed two key residues that may be critical in controlling CD38's multifunctionality of NAD hydrolysis, ADP-ribosyl cyclase, and cADPR hydrolysis activities.

CD38 is expressed selectively during the activation of a subset of mature T cells with reduced proliferation but improved potential to produce cytokines

CD38 is an approximately 45-kDa type II transmembrane glycoprotein expressed by hematopoietic and nonhematopoietic cells. Its surface expression is under complex control and varies during lymphocyte development, activation, and differentiation, suggesting an important role in these processes. Murine CD38 has been mainly characterized on B lymphocytes, and in humans, the molecule has been studied in T cells. This paper provides evidences that murine CD38 is regulated tightly during T cell activation and differentiation. On the periphery, a subset of mature T lymphocytes was identified by the expression of CD38. These cells showed an activated phenotype; they were larger and more granular than their negative counterparts. In accord with this observation, in vitro-activated T cells up-regulated CD38. Memory T lymphocytes also were CD38-positive. It is interesting that T cells expressing high levels of CD38 had a reduced, proliferative capacity but displayed an improved potential to produce interleukin-2 and interferon-gamma, suggesting a role of this molecule during T cell activation and differentiation.

Characterization and phylogenetic epitope mapping of CD38 ADPR cyclase in the cynomolgus macaque

Background

The CD38 transmembrane glycoprotein is an ADP-ribosyl cyclase that moonlights as a receptor in cells of the immune system. Both functions are independently implicated in numerous areas related to human health. This study originated from an inherent interest in studying CD38 in the cynomolgus monkey (Macaca fascicularis), a species closely related to humans that also represents a cogent animal model for the biomedical analysis of CD38.

Results

A cDNA was isolated from cynomolgus macaque peripheral blood leukocytes and is predicted to encode a type II membrane protein of 301 amino acids with 92% identity to human CD38. Both RT-PCR-mediated cDNA cloning and genomic DNA PCR surveying were possible with heterologous human CD38 primers, demonstrating the striking conservation of CD38 in these primates. Transfection of the cDNA coincided with: (i) surface expression of cynomolgus macaque CD38 by immunofluorescence; (ii) detection of ~42 and 84 kDa proteins by Western blot and (iii) the appearance of ecto-enzymatic activity. Monoclonal antibodies were raised against the cynomolgus CD38 ectodomain and were either species-specific or cross-reactive with human CD38, in which case they were directed against a common disulfide-requiring conformational epitope that was mapped to the C-terminal disulfide loop.

Conclusion

This multi-faceted characterization of CD38 from cynomolgus macaque demonstrates its high genetic and biochemical similarities with human CD38 while the immunological comparison adds new insights into the dominant epitopes of the primate CD38 ectodomain. These results open new prospects for the biomedical and pharmacological investigations of this receptor-enzyme.

Anti-HIV effects of chloroquine: inhibition of viral particle glycosylation and synergism with protease inhibitors

OBJECTIVE

We tested the effects of chloroquine (CQ) on glycosylation of HIV particles and in combination with protease inhibitors (PIs) on HIV replication and on P-glycoprotein (P-gp)/multidrug resistance protein-1 (MRP1).

DESIGN

CD4 cell lines were infected with laboratory strains and peripheral blood mononuclear cells were infected with primary isolates for evaluation of the anti-HIV effects. Peripheral blood lymphocytes were evaluated for of P-gp and MRP1 functions.

METHODS

HIV replication was assessed by enzyme-linked immunosorbent assay. HIV glycosylation was measured by metabolic labeling of viral particles with [H] glucosamine. Synergism was tested using isobolograms. P-gp and MRP1 functions were assayed using rhodamine 123 (Rh123) and carboxyfluorescein (CF) efflux assays, respectively.

RESULTS

CQ alone inhibited HIV replication and glycosylation in a dose-dependent manner. In combination with indinavir (IDV), ritonavir, or saquinavir (SQV), CQ had a synergistic effect at concentrations found in plasma of subjects receiving malaria prophylaxis. CQ decreased the 50% effective concentration of IDV in primary isolates from Africa and restored the response to IDV or SQV in 3 PI-resistant isolates. CQ increased the block of Rh123 and CF efflux activity exerted by PIs.

CONCLUSION

The inhibitory effects of CQ on HIV glycosylation are associated with synergistic effects in combination with PIs. The CQ/PI combination exerts combined inhibitory effects on P-gp and MRP1 function.

CD38 is a signaling molecule in B-cell chronic lymphocytic leukemia cells

The prognosis for patients with B-cell chronic lymphocytic leukemia (B-CLL) is generally less favorable for those expressing CD38. Our working hypothesis is that CD38 is not merely a marker in B-CLL, but that it plays a receptor role with pathogenetic potential ruling the proliferation of the malignant clone. CD38 levels were generally low in the patients examined and monoclonal antibody (mAb) ligation was inefficient in signaling. Other cellular models indicated that molecular density and surface organization are critical for CD38 functionality. Interleukin 2 (IL-2) induced a marked up-modulation and surface rearrangement of CD38 in all the patients studied. On reaching a specific expression threshold, CD38 becomes an efficient receptor in purified B-CLL cells. Indeed, mAb ligation is followed by Ca2+ fluxes and by a markedly increased proliferation. The unsuitability of CD38 to perform as a receptor is obviated through close interaction with the B-cell-receptor (BCR) complex and CD19. On mAb binding, CD38 translocates to the membrane lipid microdomains, as shown by a colocalization with the GM1 ganglioside and with CD81, a raft-resident protein. Finally, CD38 signaling in IL-2-treated B-CLL cells prolonged survival and induced the appearance of plasmablasts, providing a pathogenetic hypothesis for the occurrence of Richter syndrome.