CXCL16-dependent scavenging of oxidized lipids by islet macrophages promotes differentiation of pathogenic CD8+ T cells in diabetic autoimmunity.

The pancreatic islet microenvironment is highly oxidative, rendering ß cells vulnerable to autoinflammatory insults. Here, we examined the role of islet resident macrophages in the autoimmune attack that initiates type 1 diabetes. Islet macrophages highly expressed CXCL16, a chemokine and scavenger receptor for oxidized low-density lipoproteins (OxLDLs), regardless of autoimmune predisposition. Deletion of Cxcl16 in nonobese diabetic (NOD) mice suppressed the development of autoimmune diabetes. Mechanistically, Cxcl16 deficiency impaired clearance of OxLDL by islet macrophages, leading to OxLDL accumulation in pancreatic islets and a substantial reduction in intra-islet transitory (Texint) CD8+ T cells displaying proliferative and effector signatures. Texint cells were vulnerable to oxidative stress and diminished by ferroptosis; PD-1 blockade rescued this population and reversed diabetes resistance in NOD.Cxcl16-/- mice. Thus, OxLDL scavenging in pancreatic islets inadvertently promotes differentiation of pathogenic CD8+ T cells, presenting a paradigm wherein tissue homeostasis processes can facilitate autoimmune pathogenesis in predisposed individuals.

SHIP1 inhibition via 3-alpha-amino-cholestane enhances protection against Leishmania infection.

Leishmania major and L. donovani cause cutaneous leishmaniasis and visceral leishmaniasis, respectively. Available chemotherapies suffer from toxicity, drug-resistance or high cost of production prompting the need for the discovery of new anti-leishmanials. Here, we test a novel aminosteriodal compound- 3-alpha-amino-cholestane [3AC] - that shows selective inhibition of SHIP1, an inositol-5'-phosphate-specific phosphatase with potent effects on the immune system. We report that 3AC-sensitive SHIP1 expression increases in Leishmania-infected macrophages. Treatment of BALB/c mice, a Leishmania-susceptible host, with 3AC increased anti-leishmanial, but reduced pro-leishmanial, cytokines' production and reduced the parasite load in both L. major and L. donovani infections. These findings implicate SHIPi as a potential novel immunostimulant with anti-leishmanial function.

Obesity control by SHIP inhibition requires pan-paralog inhibition and an intact eosinophil compartment.

Here we extend the understanding of how chemical inhibition of SHIP paralogs controls obesity. We compare different classes of SHIP inhibitors and find that selective inhibitors of SHIP1 or SHIP2 are unable to prevent weight gain and body fat accumulation during increased caloric intake. Surprisingly, only pan-SHIP1/2 inhibitors (pan-SHIPi) prevent diet-induced obesity. We confirm that pan-SHIPi is essential by showing that dual treatment with SHIP1 and SHIP2 selective inhibitors reduced adiposity during excess caloric intake. Consistent with this, genetic inactivation of both SHIP paralogs in eosinophils or myeloid cells also reduces obesity and adiposity. In fact, pan-SHIPi requires an eosinophil compartment to prevent diet-induced adiposity, demonstrating that pan-SHIPi acts via an immune mechanism. We also find that pan-SHIPi increases ILC2 cell function in aged, obese mice to reduce their obesity. Finally, we show that pan-SHIPi also reduces hyperglycemia, but not via eosinophils, indicating a separate mechanism for glucose control.

LRBA Deficiency Can Lead to Lethal Colitis That Is Diminished by SHIP1 Agonism.

Humans homozygous for inactivating LRBA (lipopolysaccharide (LPS)-responsive beige-like anchor) mutations or with compound heterozygous mutations exhibit a spectrum of immune-related pathologies including inflammatory bowel disease (IBD). The cause of this pathology remains undefined. Here we show that disruption of the colon epithelial barrier in LRBA-deficient mice by dextran sulfate sodium (DSS) consumption leads to severe and uniformly lethal colitis. Analysis of bone marrow (BM) chimeras showed that susceptibility to lethal colitis is primarily due to LRBA deficiency in the immune compartment and not the gut epithelium. Further dissection of the immune defect in LRBA-deficient hosts showed that LRBA is essential for the expression of CTLA4 by Treg cells and IL22 and IL17 expression by ILC3 cells in the large intestine when the gut epithelium is compromised by DSS. We further show that SHIP1 agonism partially abrogates the severity and lethality of DSS-mediated colitis. Our findings indicate that enteropathy induced by LRBA deficiency has multiple causes and that SHIP1 agonism can partially abrogate the inflammatory milieu in the gut of LRBA-deficient hosts.

Chromogranin A Deficiency Confers Protection From Autoimmune Diabetes via Multiple Mechanisms.

Recognition of ß-cell antigens by autoreactive T cells is a critical step in the initiation of autoimmune type1 diabetes. A complete protection from diabetes development in NOD mice harboring a point mutation in the insulin B-chain 9-23 epitope points to a dominant role of insulin in diabetogenesis. Generation of NOD mice lacking the chromogranin A protein (NOD.ChgA-/-) completely nullified the autoreactivity of the BDC2.5 T cell and conferred protection from diabetes onset. These results raised the issue concerning the dominant antigen that drives the autoimmune process. Here we revisited the NOD.ChgA-/- mice and found that their lack of diabetes development may not be solely explained by the absence of chromogranin A reactivity. NOD.ChgA-/- mice displayed reduced presentation of insulin peptides in the islets and periphery, which corresponded to impaired T-cell priming. Diabetes development in these mice was restored by antibody treatment targeting regulatory T cells or inhibiting transforming growth factor-ß and programmed death-1 pathways. Therefore, the global deficiency of chromogranin A impairs recognition of the major diabetogenic antigen insulin, leading to broadly impaired autoimmune responses controlled by multiple regulatory mechanisms.

Development and Characterization of an Avirulent Leishmania major Strain.

Leishmania major causes cutaneous leishmaniasis. An antileishmanial vaccine for humans is unavailable. In this study, we report development of two attenuated L. major strains-5ASKH-HP and LV39-HP-by continuous culture (high passage) of the corresponding virulent strains (low passage). Both avirulent strains showed similar changes in proteome profiles when analyzed by surface-enhanced laser desorption ionization mass spectrometry. Liquid chromatography-mass spectrometry and microarray characterization of 5ASKH strains revealed substantially altered gene and protein expression profiles, respectively. Both virulent and avirulent L. major strains grew comparably in culture, but the avirulent strain survived significantly less in BALB/c-derived peritoneal macrophages. Both attenuated strains failed to infect BALB/c mice and elicited IFN-γ, but not IL-4 and IL-10, responses. 5ASKH-HP parasites failed to induce significant infection even in severely immunocompromised- SCID or inducible NO synthase-, CD40-, or IL-12-deficient mice, indicating attenuation. The avirulent strain induced less IL-10, but higher IL-12, in macrophages. The avirulent strain failed to reduce CD40 relocation to the detergent-resistant membrane domain and to inhibit CD40-induced phosphorylation of the kinases Lyn and protein kinase C-ß and MAPKs MKK-3/6 and p38MAPK or to upregulate MEK-1/2 and ERK-1/2 in BALB/c-derived peritoneal macrophages. The virulent and the avirulent strains reciprocally modulated CD40-induced Ras-mediated signaling through PI-3K and Raf-1. Avirulent 5ASKH-primed BALB/c mice were protected against virulent L. major challenge infection. The loss of virulence accompanied by substantially altered proteome profiles and the elicitation of host-protective immune responses indicate plausibly irreversible attenuation of the L. major strain and its potential use as a vaccine strain.

SHIP1 Deficiency in Inflammatory Bowel Disease Is Associated With Severe Crohn's Disease and Peripheral T Cell Reduction.

In our previous study, we observed a severe reduction in the Src homology 2-containing-inositol-phosphatase-1 (SHIP1) protein in a subpopulation of subjects from a small adult Crohn's Disease (CD) cohort. This pilot study had been undertaken since we had previously demonstrated that engineered deficiency of SHIP1 in mice results in a spontaneous and severe CD-like ileitis. Here, we extend our analysis of SHIP1 expression in peripheral blood mononuclear cells in a second much larger adult Inflammatory Bowel Disease (IBD) cohort, comprised of both CD and Ulcerative Colitis patients, to assess contribution of SHIP1 to the pathogenesis of human IBD. SHIP1 protein and mRNA levels were evaluated from blood samples obtained from IBD subjects seen at UCSF/SFVA, and compared to healthy control samples. Western blot analyses revealed that ~15% of the IBD subjects are severely SHIP1-deficient, with less than 10% of normal SHIP1 protein present in PBMC. Further analyses by flow cytometry and sequencing were performed on secondary samples obtained from the same subjects. Pan-hematolymphoid SHIP1 deficiency was a stable phenotype and was not due to coding changes in the INPP5D gene. A very strong association between SHIP1 deficiency and the presence of a novel SHIP1:ATG16L1 fusion transcript was seen. Similar to SHIP1-deficient mice, SHIP1-deficient subjects had reduced numbers of circulating CD4+ T cell numbers. Finally, SHIP1-deficient subjects with CD had a history of severe disease requiring multiple surgeries. These studies reveal that the SHIP1 protein is crucial for normal T cell homeostasis in both humans and mice, and that it is also a potential therapeutic and/or diagnostic target in human IBD.

LRBA is Essential for Allogeneic Responses in Bone Marrow Transplantation.

The PH-BEACH-WD40 (PBW) protein family members play a role in coordinating receptor signaling and intracellular vesicle trafficking. LPS-Responsive-Beige-like Anchor (LRBA) is a PBW protein whose immune function remains elusive. Here we show that LRBA-null mice are viable, but exhibit compromised rejection of allogeneic, xenogeneic and missing self bone-marrow grafts. Further, we demonstrate that LRBA-null Natural Killer (NK) cells exhibit impaired signaling by the key NK activating receptors, NKp46 and NKG2D. However, induction of IFN-γ by cytokines remains intact, indicating LRBA selectively facilitates signals by receptors for ligands expressed on the surface of NK targets. Surprisingly, LRBA limits immunoregulatory cell numbers in tissues where GvHD is primed or initiated, and consistent with this LRBA-null mice also demonstrate resistance to lethal GvHD. These findings demonstrate that LRBA is redundant for host longevity while being essential for both host and donor-mediated immune responses and thus represents a unique and novel molecular target in transplant immunology.

A small-molecule inhibitor of SHIP1 reverses age- and diet-associated obesity and metabolic syndrome.

Low-grade chronic inflammation is a key etiological phenomenon responsible for the initiation and perpetuation of obesity and diabetes. Novel therapeutic approaches that can specifically target inflammatory pathways are needed to avert this looming epidemic of metabolic disorders. Genetic and chemical inhibition of SH2-containing inositol 5' phosphatase 1 (SHIP1) has been associated with systemic expansion of immunoregulatory cells that promote a lean-body state; however, SHIP1 function in immunometabolism has never been assessed. This led us to investigate the role of SHIP1 in metabolic disorders during excess caloric intake in mice. Using a small-molecule inhibitor of SHIP1 (SHIPi), here we show that SHIPi treatment in mice significantly reduces body weight and fat content, improves control of blood glucose and insulin sensitivity, and increases energy expenditure, despite continued consumption of a high-fat diet. Additionally, SHIPi reduces age-associated fat in mice. We found that SHIPi treatment reverses diet-associated obesity by attenuating inflammation in the visceral adipose tissue (VAT). SHIPi treatment increases IL-4-producing eosinophils in VAT and consequently increases both alternatively activated macrophages and myeloid-derived suppressor cells. In addition, SHIPi decreases the number of IFN-γ-producing T cells and NK cells in VAT. Thus, SHIPi represents an approach that permits control of obesity and diet-induced metabolic syndrome without apparent toxicity.

Role of SHIP1 in Invariant NKT Cell Development and Functions.

SHIP1 is a 5'-inositol phosphatase known to negatively regulate the signaling product of the PI3K pathway, phosphatidylinositol (3-5)-trisphosphate. SHIP1 is recruited to a large number of inhibitory receptors expressed on invariant NK (iNKT) cells. We hypothesized that SHIP1 deletion would have major effects on iNKT cell development by altering the thresholds for positive and negative selection. Germline SHIP1 deletion has been shown to affect T cells as well as other immune cell populations. However, the role of SHIP1 on T cell function has been controversial, and its participation on iNKT cell development and function has not been examined. We evaluated the consequences of SHIP1 deletion on iNKT cells using germline-deficient mice, chimeric mice, and conditionally deficient mice. We found that T cell and iNKT cell development are impaired in germline-deficient animals. However, this phenotype can be rescued by extrinsic expression of SHIP1. In contrast, SHIP1 is required cell autonomously for optimal iNKT cell cytokine secretion. This suggests that SHIP1 calibrates the threshold of iNKT cell reactivity. These data further our understanding of how iNKT cell activation is regulated and provide insights into the biology of this unique cell lineage.

Inhibition of CD40-induced N-Ras activation reduces leishmania major infection.

Leishmania major is a parasite that resides and replicates in macrophages. We previously showed that the parasite enhanced CD40-induced Raf-MEK-ERK signaling but inhibited PI3K-MKK-p38MAPK signaling to proleishmanial effects. As Raf and PI3K have a Ras-binding domain but exert opposite effects on Leishmania infection, we examined whether Ras isoforms had differential roles in Leishmania infection. We observed that L. major enhanced N-Ras and H-Ras expression but inhibited K-Ras expression in macrophages. L. major infection enhanced N-Ras activity but inhibited H-Ras and K-Ras activity. TLR2 short hairpin RNA or anti-TLR2 or anti-lipophosphoglycan Abs reversed the L. major-altered N-Ras and K-Ras expressions. Pam3CSK4, a TLR2 ligand, enhanced N-Ras expression but reduced K-Ras expression, indicating TLR2-regulated Ras expression in L. major infection. Whereas N-Ras silencing reduced L. major infection, K-Ras and H-Ras silencing enhanced the infection both in macrophages in vitro and in C57BL/6 mice. BALB/c-derived macrophages transduced with lentivirally expressed N-Ras short hairpin RNA and pulsed with L. major-expressed MAPK10 enhanced MAPK10-specific Th1-type response. CD40-deficient mice primed with these macrophages had reduced L. major infection, accompanied by higher IFN-γ but less IL-4 production. As N-Ras is activated by Sos, a guanine nucleotide exchange factor, we modeled the N-Ras-Sos interaction and designed two peptides from their interface. Both the cell-permeable peptides reduced L. major infection in BALB/c mice but not in CD40-deficient mice. These data reveal the L. major-enhanced CD40-induced N-Ras activation as a novel immune evasion strategy and the potential for Ras isoform-targeted antileishmanial immunotherapy and immunoprophylaxis.

SHP-1 plays a crucial role in CD40 signaling reciprocity.

CD40 plays dual immunoregulatory roles in Leishmania major infection and tumor regression. The functional duality emerges from CD40-induced reciprocal p38MAPK and ERK-1/2 phosphorylations. Because phosphotyrosine-based signaling in hematopoietic cells is regulated by the phosphotyrosine phosphatase SHP-1, which is not implied in CD40 signaling, we examined whether SHP-1 played any roles in CD40-induced reciprocal signaling and anti-leishmanial function. We observed that a weaker CD40 stimulation increased SHP-1 activation. ERK-1/2 inhibition or p38MAPK overexpression inhibited CD40-induced SHP-1 activation. An ultra-low-dose, CD40-induced p38MAPK phosphorylation was enhanced by SHP-1 inhibition but reduced by SHP-1 overexpression. A reverse profile was observed with ERK-1/2 phosphorylation. SHP-1 inhibition reduced syk phosphorylation but increased lyn phosphorylation; syk inhibition reduced but lyn inhibition enhanced CD40-induced SHP-1 phosphorylation. Corroborating these findings, in L. major-infected macrophages, CD40-induced SHP-1 phosphorylation increased and SHP-1 inhibition enhanced CD40-induced p38MAPK activation and inducible NO synthase expression. IL-10 enhanced SHP-1 phosphorylation and CD40-induced ERK-1/2 phosphorylation but reduced the CD40-induced p38MAPK phosphorylation, whereas anti-IL-10 Ab exhibited reverse effects on these CD40-induced functions, identifying IL-10 as a crucial element in the SHP-1-MAPK feedback system. Lentivirally overexpressed SHP-1 rendered resistant C57BL/6 mice susceptible to the infection. Lentivirally expressed SHP-1 short hairpin RNA enhanced the CD40-induced L. major parasite killing in susceptible BALB/c mice. Thus, we establish an SHP-1-centered feedback system wherein SHP-1 modulates CD40-induced p38MAPK activation threshold and reciprocal ERK-1/2 activation, establishing itself as a critical regulator of CD40 signaling reciprocity and mechanistically re-emphasizing its role as a potential target against the diseases where CD40 is involved.

Analysis of Flow Characteristics of the Blood Flowing through an Inclined Tapered Porous Artery with Mild Stenosis under the Influence of an Inclined Magnetic Field.

Analytical investigation of MHD blood flow in a porous inclined stenotic artery under the influence of the inclined magnetic field has been done. Blood is considered as an electrically conducting Newtonian fluid. The physics of the problem is described by the usual MHD equations along with appropriate boundary conditions. The flow governing equations are finally transformed to nonhomogeneous second-order ordinary differential equations. This model is consistent with the principles of magnetohydrodynamics. Analytical expressions for the velocity profile, volumetric flow rate, wall shear stress, and pressure gradient have been derived. Blood flow characteristics are computed for a specific set of values of the different parameters involved in the model analysis and are presented graphically. Some of the obtained results show that the flow patterns in converging region (ξ < 0), diverging region (ξ > 0), and nontapered region (ξ = 0) are effectively influenced by the presence of magnetic field and change in inclination of artery as well as magnetic field. There is also a significant effect of permeability on the wall shear stress as well as volumetric flow rate.

MHD Flow of the Micropolar Fluid between Eccentrically Rotating Disks.

This analytical investigation examines the magnetohydrodynamic flow problem of an incompressible micropolar fluid between the two eccentrically placed disks. Employing suitable transformations, the flow governing partial differential equations is reduced to ordinary differential equations. An exact solution representing the different flow characteristic of micropolar fluid has been derived by solving the ordinary differential equations. Analysis of the flow characteristics of the micropolar fluid has been done graphically by varying the Reynolds number and the Hartmann number. This analysis has been carried out for the weak and strong interactions.

Role of inositol poly-phosphatases and their targets in T cell biology.

T lymphocytes play a critical role in host defense in all anatomical sites including mucosal surfaces. This not only includes the effector arm of the immune system, but also regulation of immune responses in order to prevent autoimmunity. Genetic targeting of PI3K isoforms suggests that generation of PI(3,4,5)P3 by PI3K plays a critical role in promoting effector T cell responses. Consequently, the 5'- and 3'-inositol poly-phosphatases SHIP1, SHIP2, and phosphatase and tensin homolog capable of targeting PI(3,4,5)P3 are potential genetic determinants of T cell effector functions in vivo. In addition, the 5'-inositol poly-phosphatases SHIP1 and 2 can shunt PI(3,4,5)P3 to the rare but potent signaling phosphoinositide species PI(3,4)P2 and thus these SHIP1/2, and the INPP4A/B enzymes that deplete PI(3,4)P2 may have precise roles in T cell biology to amplify or inhibit effectors of PI3K signaling that are selectively recruited to and activated by PI(3,4)P2. Here we summarize recent genetic and chemical evidence that indicates the inositol poly-phosphatases have important roles in both the effector and regulatory functions of the T cell compartment. In addition, we will discuss future genetic studies that might be undertaken to further elaborate the role of these enzymes in T cell biology as well as potential pharmaceutical manipulation of these enzymes for therapeutic purposes in disease settings where T cell function is a key in vivo target.

Modeling and experimental analyses reveals signaling plasticity in a bi-modular assembly of CD40 receptor activated kinases.

Depending on the strength of signal dose, CD40 receptor (CD40) controls ERK-1/2 and p38MAPK activation. At low signal dose, ERK-1/2 is maximally phosphorylated but p38MAPK is minimally phosphorylated; as the signal dose increases, ERK-1/2 phosphorylation is reduced whereas p38MAPK phosphorylation is reciprocally enhanced. The mechanism of reciprocal activation of these two MAPKs remains un-elucidated. Here, our computational model, coupled to experimental perturbations, shows that the observed reciprocity is a system-level behavior of an assembly of kinases arranged in two modules. Experimental perturbations with kinase inhibitors suggest that a minimum of two trans-modular negative feedback loops are required to reproduce the experimentally observed reciprocity. The bi-modular architecture of the signaling pathways endows the system with an inherent plasticity which is further expressed in the skewing of the CD40-induced productions of IL-10 and IL-12, the respective anti-inflammatory and pro-inflammatory cytokines. Targeting the plasticity of CD40 signaling significantly reduces Leishmania major infection in a susceptible mouse strain. Thus, for the first time, using CD40 signaling as a model, we show how a bi-modular assembly of kinases imposes reciprocity to a receptor signaling. The findings unravel that the signalling plasticity is inherent to a reciprocal system and that the principle can be used for designing a therapy.

Reciprocal regulation of protein kinase C isoforms results in differential cellular responsiveness.

Immunological homeostasis is often maintained by counteractive functions of two different cell types or two different receptors signaling through different intermediates in the same cell. One of these signaling intermediates is protein kinase C (PKC). Ten differentially regulated PKC isoforms are integral to receptor-triggered responses in different cells. So far, eight PKC isoforms are reported to be expressed in macrophages. Whether a single receptor differentially uses PKC isoforms to regulate counteractive effector functions has never been addressed. As CD40 is the only receptor characterized to trigger counteractive functions, we examined the relative role of PKC isoforms in the CD40-induced macrophage functions. We report that in BALB/c mouse macrophages, higher doses of CD40 stimulation induce optimum phosphorylation and translocation of PKCα, ßI, ßII, and ε whereas lower doses of CD40 stimulation activates PKCδ, ζ, and λ. Infection of macrophages with the protozoan parasite Leishmania major impairs PKCα, ßI, ßII, and ε isoforms but enhances PKCδ, ζ, and λ isoforms, suggesting a reciprocity among these PKC isoforms. Indeed, PKCα, ßI, ßII, and ε isoforms mediate CD40-induced p38MAPK phosphorylation, IL-12 expression, and Leishmania killing; PKCδ and ζ/λ mediate ERK1/2 phosphorylation, IL-10 production, and parasite growth. Treatment of the susceptible BALB/c mice with the lentivirally expressed PKCδ- or ζ-specific short hairpin RNA significantly reduces the infection and reinstates host-protective IFN-γ-dominated T cell response, defining the differential roles for PKC isoforms in immune homeostasis and novel PKC-targeted immunotherapeutic and parasite-derived immune evasion strategies.

CD40-modulated dual-specificity phosphatases MAPK phosphatase (MKP)-1 and MKP-3 reciprocally regulate Leishmania major infection.

The macrophage-expressed CD40 regulates immune responses to Leishmania major infection by reciprocal signaling through p38 MAPK and ERK1/2. CD40-induced IL-10 or IL-12 plays crucial roles in the promotion or protection from L. major infection, respectively. Because p38 MAPK and ERK1/2 are dephosphorylated by dual-specificity MAPK phosphatases (MKPs), we tested the role of CD40 in the regulation of MKPs in L. major infection. MKP-1 expression and activity increased whereas MKP-3 expression and activity decreased in virulent L. major-infected macrophages. CD40 differentially regulated the expression and activity of MKP-1 and MKP-3, which, in turn, reciprocally regulated CD40-induced p38 MAPK and ERK1/2 phosphorylation and effector functions in macrophages. Triptolide, an inhibitor of MKP-1 expression, and lentivirally expressed MKP-1 short hairpin RNA enhanced CD40-induced anti-leishmanial functions and significantly protected susceptible BALB/c mice from L. major infection. Similarly, lentivirally overexpressed MKP-3 significantly reduced disease progression and parasite burden in susceptible BALB/c mice. Thus, to our knowledge, our data show for the first time that CD40 reciprocally regulates MKP-1 and MKP-3 expression and activity while the MKPs contribute to the reciprocal CD40 signaling-regulated anti-leishmanial functions. The findings reveal a novel parasite-devised immune evasion strategy and an effective target to redirect CD40-regulated immune responses.

Leishmania interferes with host cell signaling to devise a survival strategy.

The protozoan parasite Leishmania spp. exists as extracellular promastigotes in its vector whereas it resides and replicates as amastigotes within the macrophages of its mammalian host. As a survival strategy, Leishmania modulates macrophage functions directly or indirectly. The direct interference includes prevention of oxidative burst and the effector functions that lead to its elimination. The indirect effects include the antigen presentation and modulation of T cell functions in such a way that the effector T cells help the parasite survive by macrophage deactivation. Most of these direct and indirect effects are regulated by host cell receptor signaling that occurs through cycles of phosphorylation and dephosphorylation in cascades of kinases and phosphatases. This review highlights how Leishmania selectively manipulates the different signaling pathways to ensure its survival.

Functional paradox in host-pathogen interaction dictates the fate of parasites.

The interactions between the protozoan parasite Leishmania and host macrophages are complex and involve several paradoxical functions that are meant for protection of the host but exploited by the parasite for its survival. The initial interaction of the parasite surface molecules with the host-cell receptors plays a major role in the final outcome of the disease state. While the interactions between macrophages and a virulent strain of Leishmania trigger a cascade of cell-signaling events leading to immunosuppression, the interaction with an avirulent strain triggers host-protective immune effector functions. Thus, an incisive study on Leishmania-macrophage interactions reveals functional paradoxes that highlight the concept of 'relativity in parasite virulence'. Using Leishmania infection as a model, we propose that virulence of a pathogen and the resistance (or susceptibility) of a host to the pathogen are relative properties that equate to combinatorial functions of several sets of molecular processes.