Induction of the antiviral factors APOBEC3A and RSAD2 upon CCL2 neutralization in primary human macrophages involves NF-kB, JAK/STAT, and gp130 signaling.

The CC chemokine ligand 2 (CCL2)/CC chemokine receptor 2 axis plays key roles in the pathogenesis of human immunodeficiency virus type 1 (HIV-1) infection. We previously reported that exposure of monocyte-derived macrophages (MDMs) to CCL2 neutralizing antibody (αCCL2 Ab) restricted HIV-1 replication at post-entry steps of the viral life cycle. This effect was associated with induction of transcripts coding for innate antiviral proteins, amongst which apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3A (APOBEC3A) and radical S-adenosyl methionine domain containing 2 (RSAD2). This study aimed at identifying the signaling pathways involved in induction of these factors by CCL2 blocking in MDMs. Through a combination of pharmacologic inhibition, quantitative RT-PCR, western blotting, and confocal laser-scanning microscopy, we demonstrated that CCL2 neutralization activates the canonical NF-kB and JAK/STAT pathways, as assessed by time-dependent phosphorylation of IkB, STAT1, and STAT3 and p65 nuclear translocation. Furthermore, pharmacologic inhibition of I kappa B kinase and JAKs strongly reduced APOBEC3A and RSAD2 transcript accumulation elicited by αCCL2 Ab treatment. Interestingly, exposure of MDMs to αCCL2 Ab resulted in induction of IL-6 family cytokines, and interfering with glycoprotein 130, the common signal-transducing receptor subunit shared by these cytokines, inhibited APOBEC3A and RSAD2 up-regulation triggered by CCL2 neutralization. These results provide novel insights into the signal transduction pathways underlying the activation of innate responses triggered by CCL2 neutralization in macrophages. Since this response was found to be associated with protective antiviral effects, the new findings may help design innovative therapeutic approaches targeting CCL2 to strengthen host innate immunity.

1,25(OH)2D3 Differently Modulates the Secretory Activity of IFN-DC and IL4-DC: A Study in Cells from Healthy Donors and MS Patients.

Immune mechanisms play an essential role in driving multiple sclerosis (MS) and altered trafficking and/or activation of dendritic cells (DC) were observed in the central nervous system and cerebrospinal fluid of MS patients. Interferon ß (IFNß) has been used as a first-line therapy in MS for almost three decades and vitamin D deficiency is a recognized environmental risk factor for MS. Both IFNß and vitamin D modulate DC functions. Here, we studied the response to 1,25-dihydoxyvitamin D3 (1,25(OH)2D3) of DC obtained with IFNß/GM-CSF (IFN-DC) compared to classically derived IL4-DC, in three donor groups: MS patients free of therapy, MS patients undergoing IFNß therapy, and healthy donors. Except for a decreased CCL2 secretion by IL4-DC from the MS group, no major defects were observed in the 1,25(OH)2D3 response of either IFN-DC or IL4-DC from MS donors compared to healthy donors. However, the two cell models strongly differed for vitamin D receptor level of expression as well as for basal and 1,25(OH)2D3-induced cytokine/chemokine secretion. 1,25(OH)2D3 up-modulated IL6, its soluble receptor sIL6R, and CCL5 in IL4-DC, and down-modulated IL10 in IFN-DC. IFN-DC, but not IL4-DC, constitutively secreted high levels of IL8 and of matrix-metalloproteinase-9, both down-modulated by 1,25(OH)2D3. DC may contribute to MS pathogenesis, but also provide an avenue for therapeutic intervention. 1,25(OH)2D3-induced tolerogenic DC are in clinical trial for MS. We show that the protocol of in vitro DC differentiation qualitatively and quantitatively affects secretion of cytokines and chemokines deeply involved in MS pathogenesis.

Transcriptome Profiling of Human Monocyte-Derived Macrophages Upon CCL2 Neutralization Reveals an Association Between Activation of Innate Immune Pathways and Restriction of HIV-1 Gene Expression.

Macrophages are key targets of human immunodeficiency virus type 1 (HIV-1) infection and main producers of the proinflammatory chemokine CC chemokine ligand 2 (CCL2), whose expression is induced by HIV-1 both in vitro and in vivo. We previously found that CCL2 neutralization in monocyte-derived macrophages (MDMs) strongly inhibited HIV-1 replication affecting post-entry steps of the viral life cycle. Here, we used RNA-sequencing to deeply characterize the cellular factors and pathways modulated by CCL2 blocking in MDMs and involved in HIV-1 replication restriction. We report that exposure to CCL2 neutralizing antibody profoundly affected the MDM transcriptome. Functional annotation clustering of up-regulated genes identified two clusters enriched for antiviral defense and immune response pathways, comprising several interferon-stimulated, and restriction factor coding genes. Transcripts in the clusters were enriched for RELA and NFKB1 targets, suggesting the activation of the canonical nuclear factor κB pathway as part of a regulatory network involving miR-155 up-regulation. Furthermore, while HIV-1 infection caused small changes to the MDM transcriptome, with no evidence of host defense gene expression and type I interferon signature, CCL2 blocking enabled the activation of a strong host innate response in infected macrophage cultures, and potently inhibited viral genes expression. Notably, an inverse correlation was found between levels of viral transcripts and of the restriction factors APOBEC3A (apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3 A), ISG15, and MX1. These findings highlight an association between activation of innate immune pathways and HIV-1 restriction upon CCL2 blocking and identify this chemokine as an endogenous factor contributing to the defective macrophage response to HIV-1. Therapeutic targeting of CCL2 may thus strengthen host innate immunity and restrict HIV-1 replication.

APOBEC3G/3A Expression in Human Immunodeficiency Virus Type 1-Infected Individuals Following Initiation of Antiretroviral Therapy Containing Cenicriviroc or Efavirenz.

Apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3 (APOBEC3) family members are cytidine deaminases that play crucial roles in innate responses to retrovirus infection. The mechanisms by which some of these enzymes restrict human immunodeficiency virus type 1 (HIV-1) replication have been extensively investigated in vitro. However, little is known regarding how APOBEC3 proteins affect the pathogenesis of HIV-1 infection in vivo and how antiretroviral therapy influences their expression. In this work, a longitudinal analysis was performed to evaluate APOBEC3G/3A expression in peripheral blood mononuclear cells of antiretroviral-naive HIV-1-infected individuals treated with cenicriviroc (CVC) or efavirenz (EFV) at baseline and 4, 12, 24, and 48 weeks post-treatment follow-up. While APOBEC3G expression was unaffected by therapy, APOBEC3A levels increased in CVC but not EFV arm at week 48 of treatment. APOBEC3G expression correlated directly with CD4+ cell count and CD4+/CD8+ cell ratio, whereas APOBEC3A levels inversely correlated with plasma soluble CD14. These findings suggest that higher APOBEC3G/3A levels may be associated with protective effects against HIV-1 disease progression and chronic inflammation and warrant further studies.

Dual requirement for STAT signaling in dendritic cell immunobiology.

Dendritic cells (DC) represent an attractive target for therapeutic manipulation of the immune system and enhancement of insufficient immune response in cancer. STAT family members play key roles in the differentiation and activation of DC, a feature that is currently being exploited in DC-based therapies. We previously reported that the small-molecule Stattic, originally developed as a STAT3-specific inhibitor, also inhibits STAT1 and STAT2 phosphorylation in DC exposed to cytokines or LPS. Aim of this study was to investigate the functional consequences of in vitro treatment with Stattic on DC immunobiology. Interestingly, we observed an opposite effect of Stattic on DC immunophenotype depending on the activation state. While the expression of costimulatory, coinhibitory, MHC class II and CD83 molecules was enhanced in immature DC exposed to Stattic, the LPS induced up-modulation of these molecules was strongly repressed. An effective blockade of LPS-induced secretion of proinflammatory cytokines and capacity to stimulate a Th1 polarization was also observed in the presence of Stattic. Our results indicate that the immunological consequences of STAT inhibition in DC vary depending on the cell activation state. This knowledge is of relevance for anticipating potential effects of STAT-targeted therapeutics, and pursuing selective DC manipulation in clinical applications.

Bovine Lactoferrin-Induced CCL1 Expression Involves Distinct Receptors in Monocyte-Derived Dendritic Cells and Their Monocyte Precursors.

Lactoferrin (LF) exhibits a wide range of immunomodulatory activities including modulation of cytokine and chemokine secretion. In this study, we demonstrate that bovine LF (bLF) up-modulates, in a concentration- and time-dependent manner, CCL1 secretion in monocytes (Mo) at the early stage of differentiation toward dendritic cells (DCs), and in fully differentiated immature Mo-derived DCs (MoDCs). In both cell types, up-modulation of CCL1 secretion is an early event following bLF-mediated enhanced accumulation of CCL1 transcripts. Notably, bLF-mediated up-regulation of CCL1 involves the engagement of distinct surface receptors in MoDCs and their Mo precursors. We show that bLF-mediated engagement of CD36 contributes to CCL1 induction in differentiating Mo. Conversely, toll-like receptor (TLR)2 blocking markedly reduces bLF-induced CCL1 production in MoDCs. These findings add further evidence for cell-specific differential responses elicited by bLF through the engagement of distinct TLRs and surface receptors. Furthermore, the different responses observed at early and late stages of Mo differentiation towards DCs may be relevant in mediating bLF effects in specific body districts, where these cell types may be differently represented in physiopathological conditions.

Endogenous CCL2 neutralization restricts HIV-1 replication in primary human macrophages by inhibiting viral DNA accumulation.

BACKGROUND: Macrophages are key targets of HIV-1 infection. We have previously described that the expression of CC chemokine ligand 2 (CCL2) increases during monocyte differentiation to macrophages and it is further up-modulated by HIV-1 exposure. Moreover, CCL2 acts as an autocrine factor that promotes viral replication in infected macrophages. In this study, we dissected the molecular mechanisms by which CCL2 neutralization inhibits HIV-1 replication in monocyte-derived macrophages (MDM), and the potential involvement of the innate restriction factors protein sterile alpha motif (SAM) histidine/aspartic acid (HD) domain containing 1 (SAMHD1) and apolipoprotein B mRNA-editing, enzyme-catalytic, polypeptide-like 3 (APOBEC3) family members. RESULTS: CCL2 neutralization potently reduced the number of p24 Gag+ cells during the course of either productive or single cycle infection with HIV-1. In contrast, CCL2 blocking did not modify entry of HIV-1 based Virus Like Particles, thus demonstrating that the restriction involves post-entry steps of the viral life cycle. Notably, the accumulation of viral DNA, both total, integrated and 2-LTR circles, was strongly impaired by neutralization of CCL2. Looking for correlates of HIV-1 DNA accumulation inhibition, we found that the antiviral effect of CCL2 neutralization was independent of the modulation of SAMHD1 expression or function. Conversely, a strong and selective induction of APOBEC3A expression, to levels comparable to those of freshly isolated monocytes, was associated with the inhibition of HIV-1 replication mediated by CCL2 blocking. Interestingly, the CCL2 neutralization mediated increase of APOBEC3A expression was type I IFN independent. Moreover, the transcriptome analysis of the effect of CCL2 blocking on global gene expression revealed that the neutralization of this chemokine resulted in the upmodulation of additional genes involved in the defence response to viruses. CONCLUSIONS: Neutralization of endogenous CCL2 determines a profound restriction of HIV-1 replication in primary MDM affecting post-entry steps of the viral life cycle with a mechanism independent of SAMHD1. In addition, CCL2 blocking is associated with induction of APOBEC3A expression, thus unravelling a novel mechanism which might contribute to regulate the expression of innate intracellular viral antagonists in vivo. Thus, our study may potentially lead to the development of new therapeutic strategies for enhancing innate cellular defences against HIV-1 and protecting macrophages from infection.

Increased circulating levels of vitamin D binding protein in MS patients.

Vitamin D (vitD) low status is currently considered a main environmental factor in multiple sclerosis (MS) etiology and pathogenesis. VitD and its metabolites are highly hydrophobic and circulate mostly bound to the vitamin D binding protein (DBP) and with lower affinity to albumin, while less than 1% are in a free form. The aim of this study was to investigate whether the circulating levels of either of the two vitD plasma carriers and/or their relationship are altered in MS. We measured DBP and albumin plasma levels in 28 MS patients and 24 healthy controls. MS patients were found to have higher DBP levels than healthy subjects. Concomitant interferon beta therapy did not influence DBP concentration, and the difference with the control group was significant in both females and males. No significant correlation between DBP and albumin levels was observed either in healthy controls or in patients. These observations suggest the involvement of DBP in the patho-physiology of MS.

STAT3-silenced human dendritic cells have an enhanced ability to prime IFNγ production by both αß and γδ T lymphocytes.

Dendritic cells (DC) are an attractive target for therapeutic manipulation of the immune system to enhance insufficient immune responses, such those occurring in cancer, or to dampen dangerous responses in allergic and autoimmune diseases. Main goal of this study was to manipulate human monocyte-derived DC (MDDC) function by silencing STAT3, since this transcription factor plays a key role as a negative regulator of immune surveillance, and is strongly involved in inflammation. STAT3 silencing did not affect the immunophenotype of both immature and toll-like receptor (TLR) ligand-matured DC. However, an altered cytokine secretion profile, characterized by lower IL10 and higher IL12 and TNFα levels, was observed in silenced DC with respect to control cells upon TLR triggering. Accordingly, STAT3 silenced MDDC promoted a higher IFNγ production by CD4(+) naïve T cells. Furthermore, STAT3 silencing in MDDC favored the activation of γδ T lymphocytes, an immune cell population with important antitumor effector activities. This effect was at least in part mediated by the increased IL12 production by silenced cells. STAT3 silencing also increased the levels of CCL4, a CCR5-binding chemokine known to be involved in T helper 1 (Th1) cell recruitment. Altogether these results strengthen the role of STAT3 as a critical check point of the suppression of Th1 responses, unraveling its potential to dampen DC capability to both induce and recruit different IFNγ producing T lymphocyte subsets.

CCL2 induction by 1,25(OH)2D3 in dendritic cells from healthy donors and multiple sclerosis patients.

CCL2 plays a pivotal role in the recruitment of different immune cells to sites of inflammation and evidence indicates its involvement in multiple sclerosis (MS) pathogenesis. MS lesions are characterized by an inflammatory infiltrate, whose nature is controlled by chemokines and cytokines, and elevated expression of CCL2 has been found in acute and chronic MS plaques within the brain. Vitamin D deficiency is currently considered one of the main environmental MS risk factors. In this study we analyzed the role of 1,25(OH)2D3, the bioactive vitamin D metabolite, in the regulation of CCL2 expression by dendritic cells (DC) obtained from healthy donors and relapsing-remitting MS patients. We report that 1,25(OH)2D3, as well as 25OHD3, its main blood precursor, induce the secretion of high levels of CCL2. 1,25(OH)2D3-induced CCL2 levels are comparable to those secreted in response to a classical DC maturation stimulus. Moreover, we observed that 1,25(OH)2D3 is able to induce a significant CCL2 secretion in DC obtained from relapsing-remitting MS patients, although CCL2 levels in these latter are lower with respect to healthy controls. The cause(s) of this apparently defective response of DC from patients and its consequences in the context of MS remain to be elucidated. However, we propose CCL2 as a molecular player contributing to the immunomodulatory activity of 1,25(OH)2D3 on DC, and hypothesize a role for this chemokine in the response of MS patients to vitamin D therapy. This article is part of a Special Issue entitled '16th Vitamin D Workshop'.

Nuclear phosphoinositide-specific phospholipase C ß1 controls cytoplasmic CCL2 mRNA levels in HIV-1 gp120-stimulated primary human macrophages.

HIV-1 envelope glycoprotein gp120 induces, independently of infection, the release of CCL2 from macrophages. In turn, this chemokine acts as an autocrine factor enhancing viral replication. In this study, we show for the first time that phosphoinositide-specific phospholipase C (PI-PLC) is required for the production of CCL2 triggered by gp120 in macrophages. Using a combination of confocal laser-scanner microscopy, pharmacologic inhibition, western blotting and fluorescence-activated cell sorter analysis, we demonstrate that gp120 interaction with CCR5 leads to nuclear localization of the PI-PLC ß1 isozyme mediated by mitogen-activated protein kinase ERK-1/2. Notably, phosphatidylcholine-specific phospholipase C (PC-PLC), previously reported to be required for NF-kB-mediated CCL2 production induced by gp120 in macrophages, drives both ERK1/2 activation and PI-PLC ß1 nuclear localization induced by gp120. PI-PLC ß1 activation through CCR5 is also triggered by the natural chemokine ligand CCL4, but independently of ERK1/2. Finally, PI-PLC inhibition neither blocks gp120-mediated NF-kB activation nor overall accumulation of CCL2 mRNA, whereas it decreases CCL2 transcript level in the cytoplasm. These results identify nuclear PI-PLC ß1 as a new intermediate in the gp120-triggered PC-PLC-driven signal transduction pathway leading to CCL2 secretion in macrophages. The finding that a concerted gp120-mediated signaling involving both PC- and PI-specific PLCs is required for the expression of CCL2 in macrophages suggests that this signal transduction pathway may also be relevant for the modulation of viral replication in these cells. Thus, this study may contribute to identify novel targets for therapeutic intervention in HIV-1 infection.

Toll-like receptor cross-talk in human monocytes regulates CC-chemokine production, antigen uptake and immune cell recruitment.

Chemokines production in monocytes/macrophages is crucial in modulating immune responses generated through Toll-like receptor (TLR)-mediated recognition of microbes. During microbial onset, multiple pathogen-associated structures can be present at infection sites, and simultaneously trigger different TLRs. We report here that TLR3, TLR4 and TLR8 engagement induce CCL1, CCL2 and CCL4 production in freshly isolated monocytes. While differentiating cells maintain the capacity to secrete CCL2 and CCL4, CCL1 is no longer induced at later differentiation stages. Although different pairs of TLR agonists have been described to synergistically induce cytokine production in different cell types, agonist combinations cooperate in reducing CCL1 and CCL2, but not CCL4 secretion in freshly isolated monocytes, and fail to rescue CCL1 production at later differentiation stages. The effects of single, but not combined, TLR engagement on chemokine expression mostly occur at transcriptional level, and are IL-10 independent. Conversely, inhibition of CCL1 secretion upon combined TLR engagement is partially rescued by blocking IL-23. A different chemotactic activity of monocyte-conditioned medium on blood mononuclear cells as well as antigen uptake capacity of TLR agonist activated monocytes parallel the regulated production of chemokines. Overall, these findings indicate that simultaneous engagement of TLRs may lead to different patterns of chemokine expression depending on cellular differentiation state, chemokine, and TLR agonist combination. These different responses may be relevant for the distinct but complementary functions of monocytes and macrophages in the immune response, and may have important implications for the therapeutic manipulation of the innate immune system.

Phosphatidylcholine-specific phospholipase C activation is required for CCR5-dependent, NF-kB-driven CCL2 secretion elicited in response to HIV-1 gp120 in human primary macrophages.

CCL2 (MCP-1) has been shown to enhance HIV-1 replication. The expression of this chemokine by macrophages is up-modulated as a consequence of viral infection or gp120 exposure. In this study, we show for the first time that the phosphatidylcholine-specific phospholipase C (PC-PLC) is required for the production of CCL2 triggered by gp120 in human monocyte-derived macrophages (MDMs). Using a combination of pharmacologic inhibition, confocal laser-scanner microscopy, and enzymatic activity assay, we demonstrate that R5 gp120 interaction with CCR5 activates PC-PLC, as assessed by a time-dependent modification of its subcellular distribution and a concentration-dependent increase of its enzymatic activity. Furthermore, PC-PLC is required for NF-kB-mediated CCL2 production triggered by R5 gp120. Notably, PC-PLC activation through CCR5 is specifically induced by gp120, since triggering CCR5 through its natural ligand CCL4 (MIP-1beta) does not affect PC-PLC cellular distribution and enzymatic activity, as well as CCL2 secretion, thus suggesting that different signaling pathways can be activated through CCR5 interaction with HIV-1 or chemokine ligands. The identification of PC-PLC as a critical mediator of well-defined gp120-mediated effects in MDMs unravels a novel mechanism involved in bystander activation and may contribute to define potential therapeutic targets to block Env-triggered pathologic responses.

IRF-4 expression in the human myeloid lineage: up-regulation during dendritic cell differentiation and inhibition by 1alpha,25-dihydroxyvitamin D3.

Interferon (IFN) regulatory factor (IRF)-4 is a lymphoid- and myeloid-restricted transcription factor of the IRF family. We analyzed its expression during differentiation of human monocytes along the macrophage or the dendritic cell (DC) pathway and in blood myeloid and plasmacytoid DC (M-DC and P-DC, respectively) subsets. Monocyte differentiation into DC, driven by granulocyte macrophage-colony stimulating factor (GM-CSF)/interleukin-4 or GM-CSF/IFN-beta, resulted in a strong up-regulation of IRF-4 mRNA and protein, which was further increased by lipopolysaccharide. It is interesting that 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], a potent inhibitor of DC differentiation, completely abolished IRF-4 up-regulation. IRF-4 was also detected in blood P-DC and M-DC. However, up-regulation upon in vitro culture and down-regulation by 1,25(OH)(2)D(3) was observed in M-DC but not in P-DC. These results point to IRF-4 as a potential player in human myeloid DC differentiation and as a novel target for the immunomodulatory activity of 1,25(OH)(2)D(3).

Suppressive effect of 1alpha,25-dihydroxyvitamin D3 on type I IFN-mediated monocyte differentiation into dendritic cells: impairment of functional activities and chemotaxis.

Dendritic cells (DCs) generated by a single-step exposure of human monocytes to type I IFN and GM-CSF (IFN-DCs) are endowed with potent immunostimulatory activities and a distinctive migratory response to specific chemokines. In this study, we evaluated the effects of 1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), the biologically active metabolite of vitamin D(3), on the DC differentiation/activation induced by type I IFN. We found that 1,25(OH)(2)D(3) prevented the generation of IFN-DCs when added to freshly isolated monocytes, and was capable of redirecting already differentiated IFN-DCs toward a more immature stage, as revealed by their immunophenotype, reduced allostimulatory activity, and impaired LPS-induced production of Th1-polarizing cytokines. Control and 1,25(OH)(2)D(3)-treated IFN-DCs exhibited a similar expression of vitamin D receptor, as well as comparable cell death rates. Furthermore, the chemotactic response of IFN-DCs to CCL4 and CCL19 was markedly reduced or completely abrogated by 1,25(OH)(2)D(3). Despite these changes in the IFN-DC migratory behavior, the expression of CCR5 and CCR7 and the calcium fluxes triggered by CCL4 and CCL19 were not affected. These findings indicate that, in this innovative single-step DC generation model from monocytes, the suppressive effect of 1,25(OH)(2)D(3) is associated with a potent impairment of DC migration in response to inflammatory and lymph node-homing chemokines, thus unraveling a novel mechanism involved in 1,25(OH)(2)D(3)-mediated immunomodulation.

Human immunodeficiency virus type 1 gp120 induces abnormal maturation and functional alterations of dendritic cells: a novel mechanism for AIDS pathogenesis.

Dendritic cells (DCs) play a crucial role in bridging innate and acquired immune responses to pathogens. In human immunodeficiency virus type 1 (HIV-1) infection, immature DCs (iDCs) are also main targets for HIV-1 at the mucosal level. In this study, we evaluated the effects of HIV-1-DC interactions on the maturation and functional activity of these cells. Exposure of human monocyte-derived iDCs to either aldrithiol-2-inactivated HIV-1 or gp120 led to an upmodulation of activation markers indicative of functional maturation. Despite their phenotype, these cells retained antigen uptake capacity and showed an impaired ability to secrete cytokines or chemokines and to induce T-cell proliferation. Although gp120 did not interfere with DC differentiation, the capacity of these cells to produce interleukin-12 (IL-12) upon maturation was markedly reduced. Likewise, iDCs stimulated by classical maturation factors in the presence of gp120 lacked allostimulatory capacity and did not produce IL-12, in spite of their phenotype typical of activated DCs. Exogenous addition of IL-12 restores the allostimulatory capacity of gp120-exposed DCs. The finding that gp120 induces abnormal maturation of DCs linked to profound suppression of their activities unravels a novel mechanism by which HIV can lead to immune dysfunction in AIDS patients.

Malignant pancreatic endocrine tumor in a child with tuberous sclerosis.

Tuberous sclerosis complex (TSC) is an autosomal dominant condition whose signs and symptoms may vary from a few hypopigmented skin spots to epilepsy, severe mental retardation, and renal failure. The disease is caused by mutations in either TSC1 or TSC2 gene, at chromosome 9q34 and 16p13.3. Inactivation of both alleles at TSC1 or TSC2 loci is associated with the development of hamartomas in different organs, and only rarely with malignant neoplasms. In this study we present a 6-year-old boy with TSC and with a malignant islet cell tumor of the pancreas. Mutation analysis of DNA extracted from peripheral blood cells of the patient identified an R1459X de novo mutation in exon 33 of the TSC2 gene. Immunohistochemical analysis with anti-tuberin antibodies on paraffin-embedded tissue sections showed loss of tuberin immunostaining in tumor cells but normal expression in residual normal pancreas. DNA analysis of tumor and normal cells showed chromosome 16p13 loss of heterozygosity in malignant pancreatic islet cell tumor but not in normal pancreas. These findings suggest a role for tuberin, the TSC2 gene product, in the pathogenesis of malignant pancreatic endocrine tumor.