Importance of Cellular Microenvironment and Circulatory Dynamics in B Cell Immunotherapy

B cell immunotherapy has emerged as a mainstay in the treatment of lymphomas and autoimmune diseases. Although the microenvironment has recently been demonstrated to play critical roles in B cell homeostasis, its contribution to immunotherapy is unknown. To analyze the in vivo factors that regulate mechanisms involved in B cell immunotherapy, we used a murine model for human CD20 (hCD20) expression in which treatment of hCD20(+) mice with anti-hCD20 mAbs mimics B cell depletion observed in humans. We demonstrate in this study that factors derived from the microenvironment, including signals from the B cell-activating factor belonging to the TNF family/BLyS survival factor, integrin-regulated homeostasis, and circulatory dynamics of B cells define distinct in vivo mechanism(s) and sensitivities of cells in anti-hCD20 mAb-directed therapies. These findings provide new insights into the mechanisms of immunotherapy and define new opportunities in the treatment of cancers and autoimmune diseases.

Plasma fibronectin (opsonic glycoprotein): its synthesis by vascular endothelial cells and role in cardiopulmonary integrity after trauma as related to reticuloendothelial function

Progressive multiple organ failure in association with septic complications in the surgical, trauma and burn patient is of major clinical importance. Reticuloendothelial system host defense mechanisms are abnormal following severe trauma and burn. Failure in systemic host defense is, in part, mediated by a deficiency in a circulating opsonic alpha 2 surface binding (SB) glycoprotein. This opsonic deficiency and reticuloendothelial host defense failure appears etiologic in the genesis of organ failure with sepsis. Opsonic alpha 2SB glycoprotein is identical to cold-insoluble globulin or plasma fibronectin. Plasma fibronectin is antigenically related to cell surface fibronectin which appears to be synthesized by both fibroblasts and vascular endothelial cells. Although these two proteins are antigenically related, they may or may not be identical with respect to biochemical properties and function. Cell surface fibronectin appears to be an adhesive glycoprotein mediating cell-cell interaction and cell adhesion to a substratum. Plasma fibronectin is a more soluble form which mediates reticuloendothelial or macrophage clearance of particulates such as fibrin microaggregates, collagenous debris, perhaps other bacterial or nonbacterial particulates. Since opsonic glycoprotein is identical to cold-insoluble globulin which can be readily concentrated in plasma cryoprecipitate, it has been shown that cryoprecipitate infusion can reverse opsonic deficiency in the injured patient with sepsis. Reversal of opsonic deficiency by cryoprecipitate infusion results in a marked improvement in cardiopulmonary function which includes a decline in the pulmonary shunt, a decrease in the physiologic dead space, an increase in limb blood flow, an increase in reactive hyperemia of the peripheral circulation and an increase in limb oxygen consumption. This cardiopulmonary response is paralleled by a decline in the septic state and normalization of other hematologic parameters. These studies suggest an important homeostatic role for fibronectins in organ and microvascular integrity, especially in the septic injured patient. Cell surface fibronectin which participates in cell adhesion may, in part, modulate microvascular integrity, vascular permeability and would repair. In contrast, the more soluble plasma fibronectin or opsonic alpha 2SB glycoprotein may mediate reticuloendothelial clearance of blood-borne particulates to prevent pulmonary and peripheral vascular microembolization and organ injury. Thus, reversal of opsonic deficiency may be an effective modality of therapy in the septic injured patient with organ failure.

Sjögren's syndrome (Sicca syndrome): current issues

This paper outlines the clinical, serologic, and immunogenetic differences and similarities of Sjögren's syndrome alone (primary) and Sjögren's syndrome associated with rheumatoid arthritis and systemic lupus erythematosus (secondary). The immunoregulation in Sjögren's syndrome is discussed and the incidence of immune complex-like material, its nature, pathophysiology, and clearance by the Fc recptor of the reticuloendothelial system presented.

Defective reticuloendothelial system Fc-receptor function in systemic lupus erythematosus

To determine whether reticuloendothelial-system immunospecific Fc-receptor function is abnormal in patients with systemic lupus erythematosus, we studied the clearance of IgG-sensitized 51Cr-labeled erythrocytes by these splenic macrophage membrane receptors in 15 untreated patients. Fc-specific clearance rates were strikingly abnormal in 13 of 15 patients (half-times ranging from 80 to 2256 minutes, P less than 0.001 as compared to controls). Abnormal clearances correlated with immune-complex levels (as measured by the C1q-binding assay) and with disease activity. C1q-binding activity and anti-DNA titers also correlated with disease activity. The correlations of C3, C4, CH50 and factor B with abnormal clearance and disease activity were weaker or nonexistent. The significant correlations among clearance, disease activity and C1q-binding activity suggest that the defect in Fc-receptor function may lead to the prolonged circulation of immune complexes, thereby contributing to tissue deposition and damage.

Reticuloendothelial system phagocytic activity in cirrhosis and its relation to bacterial infections and prognosis

The reticuloendothelial system phagocytic activity, estimated by the plasma elimination rate constant of 99mtechnetium-sulfur colloid, was studied in 41 decompensated cirrhotics and 10 normal subjects. The results were related to the incidence and type of bacterial infections occurring during hospitalization and follow-up, and to survival. The elimination rate constant of 99mtechnetium-sulfur colloid was lower in cirrhotic patients (0.168 +/- 0.007) (means +/- S.E.) than in normal subjects (0.220 +/- 0.005) (p less than 0.01). Cirrhotics were divided into two groups. Group I (16 patients) and Group II (25 patients) had normal or reduced elimination rate constant of 99mtechnetium-sulfur colloid, respectively. Both groups were similar in relation to clinical and biochemical data, hepatic blood flow, and wedged hepatic venous pressure. However, the liver scan and the elimination rate constant of indocyanine green were more altered in Group II. Patients in Group II developed acute bacterial infections more frequently than did patients in Group I. During hospitalization (24 +/- 2 days), bacteremia occurred in six patients in Group II and in none in Group I (p less than 0.05). During follow-up (28 +/- 3 months), 5 patients in Group II and none in Group I developed bacteremia (p less than 0.05). The cumulative survival rate of Group I patients was higher (p less than 0.05) than that of Group II patients at 3 months (100 vs. 80%), 6 months (94 vs. 68%), 24 months (74 vs. 42%), and 48 months (68 vs. 34%). We suggest that decompensated cirrhotics with depressed reticuloendothelial system phagocytic activity are at great risk to acquire bacteremia, and that reticuloendothelial system phagocytic activity has prognostic value in cirrhosis.

The role of melatonin in the cells of the innate immunity: a review

Melatonin is the major secretory product synthesized and secreted by the pineal gland and shows both a wide distribution within phylogenetically distant organisms from bacteria to humans and a great functional versatility. In recent years, a considerable amount of experimental evidence has accumulated showing a relationship between the nervous, endocrine, and immune systems. The molecular basis of the communication between these systems is the use of a common chemical language. In this framework, currently melatonin is considered one of the members of the neuroendocrine-immunological network. A number of in vivo and in vitro studies have documented that melatonin plays a fundamental role in neuroimmunomodulation. Based on the information published, it is clear that the majority of the present data in the literature relate to lymphocytes; thus, they have been rather thoroughly investigated, and several reviews have been published related to the mechanisms of action and the effects of melatonin on lymphocytes. However, few studies concerning the effects of melatonin on cells belonging to the innate immunity have been reported. Innate immunity provides the early line of defense against microbes and consists of both cellular and biochemical mechanisms. In this review, we have focused on the role of melatonin in the innate immunity. More specifically, we summarize the effects and action mechanisms of melatonin in the different cells that belong to or participate in the innate immunity, such as monocytes-macrophages, dendritic cells, neutrophils, eosinophils, basophils, mast cells, and natural killer cells.

The lymphoreticular system in triggering virus plus self-specific cytotoxic T cells: evidence for T help

The thymus determines the spectrum of the receptor specificities of differentiating T cells for self-H-2; however, the phenotypic expression of T cell's specificity for self plus virus is determined predominantly by the H-2 type of the antigen presenting cells of the peripheral lymphoreticular system. Furthermore, virus specific helper T cells are essential for the generation of virus-specific cytotoxic T cells. For cooperation between mature T cells and other lymphocytes to be functional in chimeras, thymic epithelial cells and lymphohemopoietic stem cells must share the I region; killer T-cell generation also requires in addition compatibility for at least one K or D region. These conclusions derive from the following experiments: A leads to (A X B)F1 chimeric lymphocytes do produce virus-specific cytotoxic T-cell activity for infected A but not for infected B cells; when sensitized in an acutely irradiated and infected recipient (A X B)F1 these chimeric lymphocytes respond to both infected A and B. Therefore the predominantly immunogenically infected cells of chimeras the radiosensitive and by donor stem cells replaced lymphoreticular cells. In this adoptive priming model (KAIA/DB leads to KAIA/DC) chimeric lymphocytes could be sensitized in irradiated and infected F1 against KA and DC but not against infected DB targets. In contrast KBIB/DA leads to KCIC/DA chimeras' lymphocytes could not be sensitized at all in appropriately irradiated and infected F1 recipients. Thus these latter chimeras probably lack functional I-specific T helper cells that are essential for the generation of T killer cells against infected D compatible targets. If T cells learn in the thymus to recognize H-21 or K, D markers that are not at least partially carried themselves in other cells of the lymphoreticular system immunological interactions will be impossible and this paradox situation results in phenotypic immune incompetence in vivo.

Host defenses and immunologic alterations associated with chronic hemodialysis

Many of the complications experienced by patients undergoing hemodialysis can be attributed to their altered host defenses. Increased cutaneous staphylococcal carriage along with repeated intravascular cannulation and defective mucocutaneous barriers lead to frequent invasion by infectious agents. Pathogens encounter granulocytes with subnormal locomotion, phagocytosis, and intracellular killing. Depressed cell-mediated immunity may be explained by shortened lymphocyte survival, lymphopenia, inhibition of lymphocyte transformation, and suppressor T-cell activity. This is manifested by cutaneous anergy, prolonged graft survival, altered tumor surveillance, and abnormal responses to hepatitis B and tuberculosis. Host interaction with the hemodialysis membrane leads to cellular disruption, which may induce autoantibodies. Activation of the alternate complement pathway during hemodialysis leads to granulocyte sequestration in small vessels, specifically within the lungs. These hemodialysis-induced alterations along with the manifestations of underlying chronic renal insufficiency may obscure clinical evaluation of these patients.

Overcoming the Reticuloendothelial System Barrier to Drug Delivery with a "Don't-Eat-Us" Strategy

Overcoming the reticuloendothelial system (RES) has long been a vital challenge to nanoparticles as drug carriers. Modification of nanoparticles with polyethylene glycol helps them avoid clearance by macrophages but also suppresses their internalization by target cells. To overcome this paradox, we developed an RES-specific blocking system utilizing a "don't-eat-us" strategy. First, a CD47-derived, enzyme-resistant peptide ligand was designed and placed on liposomes (d-self-peptide-labeled liposome, DSL). After mainline administration, DSL was quickly adsorbed onto hepatic phagocyte membranes (including those of Kupffer cells and liver sinusoidal endothelial cells), forming a long-lasting mask that enclosed the cell membranes and thus reducing interactions between phagocytes and subsequently injected nanoparticles. Compared with blank conventional liposomes (CL), DSL blocked the RES at a much lower dose, and the effect was sustained for a much longer time, highly prolonging the elimination half-life of the subsequently injected nanoparticles. This "don't-eat-us" strategy by DSL was further verified on the brain-targeted delivery against a cryptococcal meningitis model, providing dramatically enhanced brain accumulation of the targeted delivery system and superior therapeutic outcome of model drug Amphotericin B compared with CL. Our study demonstrates a strategy that blocks the RES by masking phagocyte surfaces to prolong nanoparticle circulation time without excess modification and illustrates its utility in enhancing nanoparticle delivery.

Effect of perorally administered lactobacilli on macrophage activation in mice

The effect of perorally (p.o) administered Lactobacillus casei and L. bulgaricus on macrophage activation in mice was studied. L. casei and L. bulgaricus were administered p.o. to mice for 8 days. The macrophage activation was measured on days 2, 3, 5, and 8 of lactobacillus administration by using biochemical and functional criteria. We measured the release of lysosomal hydrolases, the level of a nonlysosomal enzyme, and in vitro phagocytic activity of mouse peritoneal macrophages. All the assays were performed comparatively with mice inoculated with L. casei and L. bulgaricus (viable and nonviable cells) intraperitoneally (i.p.) at the same dose as for p.o. administration. The phagocytic activity was significantly higher in mice treated i.p. than in control mice. For p.o. administration, there was an increase only when L. casei was used. L. bulgaricus had little effect. No differences were found between viable and nonviable cells. The phagocytic function of the reticuloendothelial system was tested by the carbon clearance test, which showed that L. casei and L. bulgaricus accelerate the phagocytic function in mice treated p.o and i.p., from day 2 onward. These observations show that L. casei and L. bulgaricus given by p.o. administration are able to activate macrophages in mice and suggest that these bacteria, when passing through the intestinal tract, may be responsible for the enhanced host immune response. This fact is very significant because the diet includes fermented and manufactured products containing lactobacilli.

Cryoprecipitate reversal of opsonic alpha2-surface binding glycoprotein deficiency in septic surgical and trauma patients

Human opsonic alpha2-surface binding glyoprotein (alphs2SB-glycoprotein), a molecule having immunologic identity with an amino acid composition similar to cold-insoluble globulin, is concentrated in a cryoprecipitate of plasma. Septic surgical and trauma patients manifesting opsonic alpha2SB-glycoprotein deficiency and associated reticuloendothelial system dysfunction were treated by intravenous infusion of cryoprecipitate. This therapy restored circulating bioreactive and immunoreactive opsonin and improved their septicemia, pulmonary insufficiency, and duration of recovery. Cryoprecipitate infusion may offer a new approach to the treatment of septic injured patients in preventing multiple organ failure; measurement of immuno-reactive serum opsonic alpha2SB-glycoprotein may provide a noninvasive index of reticuloendothelial system function and patient status during servere sepsis that follows trauma.

Studies on antigen-antibody complexes. I. Elimination of soluble complexes from rabbit circulation

Solid phase immunoadsorbents were prepared by coupling antigens to agarose. With this technique specific antibodies were easily isolated in large amounts. The gammaG-globulin class of antibodies isolated in this manner were not denatured as judged by their normal biological half-life in rabbits. Soluble immune complexes at fivefold antigen excess were prepared from isolated specific antibodies and HSA, human lambda-chains, human lambdaG-globulins, and a Waldenström's macroglobulin as antigens. In all these preparations a characteristic immune complex was encountered that represented the smallest stable antigen-antibody union. In the HSA-anti-HSA system they were found to be AgAb(2) complexes, and Ag(2)Ab complexes in the gammaG-anti-gammaG system. These stable complexes fixed complement ineffectively. Also, a spectrum of larger complexes was present in each system, and these complexes fixed complement effectively. With intact antibodies the disappearance curves of immune complexes from the circulation were composed of three exponential components. The immune complexes larger than AgAb(2) were quickly removed from the circulation with half-lives of 0.09-0.37 hr. Their clearance was not dependent on complement components, in that depletion of complement by cobra venom factor and aggregated gammaG-globulin did not alter the pattern of their removal from the circulation. However, when the interchain disulfide bonds of antibodies were reduced and alkylated, the removal of the lambda-anti-lambda, HSA-anti-HSA, and gammaG-anti-gammaG complexes was altered. In these experiments the disappearance curves were composed of two exponential components and the rapid removal of the greater than AgAb(2) complexes did not occur. The immune complexes prepared from reduced and alkylated antibodies fixed complement ineffectively. The presented data indicate that the rapid removal of circulating immune complexes, containing gammaG-globulin molecules as antibodies, depends primarily on the number of antibodies involved. Furthermore, complement fixation is not involved in the rapid removal of such complexes. Nevertheless, the rapid removal of immune complexes and their ability to fix complement have similarities for optimal function in that both processes require intact interchain disulfide bonds of antibodies and complexes that exceed the AgAb(2) combination.

Critical Role for the Microbiota in CX3CR1+ Intestinal Mononuclear Phagocyte Regulation of Intestinal T Cell Responses

The intestinal barrier is vulnerable to damage by microbiota-induced inflammation that is normally restrained through mechanisms promoting homeostasis. Such disruptions contribute to autoimmune and inflammatory diseases including inflammatory bowel disease. We identified a regulatory loop whereby, in the presence of the normal microbiota, intestinal antigen-presenting cells (APCs) expressing the chemokine receptor CX3CR1 reduced expansion of intestinal microbe-specific T helper 1 (Th1) cells and promoted generation of regulatory T cells responsive to food antigens and the microbiota itself. We identified that disruption of the microbiota resulted in CX3CR1+ APC-dependent inflammatory Th1 cell responses with increased pathology after pathogen infection. Colonization with microbes that can adhere to the epithelium was able to compensate for intestinal microbiota loss, indicating that although microbial interactions with the epithelium can be pathogenic, they can also activate homeostatic regulatory mechanisms. Our results identify a cellular mechanism by which the microbiota limits intestinal inflammation and promotes tissue homeostasis.

Flow Cytometric Analysis of Mononuclear Phagocytes in Nondiseased Human Lung and Lung-Draining Lymph Nodes

RATIONALE

The pulmonary mononuclear phagocyte system is a critical host defense mechanism composed of macrophages, monocytes, monocyte-derived cells, and dendritic cells. However, our current characterization of these cells is limited because it is derived largely from animal studies and analysis of human mononuclear phagocytes from blood and small tissue resections around tumors.

OBJECTIVES

Phenotypic and morphologic characterization of mononuclear phagocytes that potentially access inhaled antigens in human lungs.

METHODS

We acquired and analyzed pulmonary mononuclear phagocytes from fully intact nondiseased human lungs (including the major blood vessels and draining lymph nodes) obtained en bloc from 72 individual donors. Differential labeling of hematopoietic cells via intrabronchial and intravenous administration of antibodies within the same lobe was used to identify extravascular tissue-resident mononuclear phagocytes and exclude cells within the vascular lumen. Multiparameter flow cytometry was used to identify mononuclear phagocyte populations among cells labeled by each route of antibody delivery.

MEASUREMENTS AND MAIN RESULTS

We performed a phenotypic analysis of pulmonary mononuclear phagocytes isolated from whole nondiseased human lungs and lung-draining lymph nodes. Five pulmonary mononuclear phagocytes were observed, including macrophages, monocyte-derived cells, and dendritic cells that were phenotypically distinct from cell populations found in blood.

CONCLUSIONS

Different mononuclear phagocytes, particularly dendritic cells, were labeled by intravascular and intrabronchial antibody delivery, countering the notion that tissue and blood mononuclear phagocytes are equivalent systems. Phenotypic descriptions of the mononuclear phagocytes in nondiseased lungs provide a precedent for comparative studies in diseased lungs and potential targets for therapeutics.

The primary role of lymphoreticular cells in the mediation of host responses to bacterial endotoxim

Mice that are unresponsive to lipopolysaccharide (LPS) (strain C3H/HeJ) can be rendered LPS-sensitive by the adoptive transfer of bone marrow cells from LPS-sensitive mice (strain C3H/HeN). This model of adoptive transfer was used to evaluate the contribution of lymphoreticular cells to five effects of endotoxin on the host: immunogenicity, adjuvanticity, lethality, induction of interferon, and induction of colony-stimulated factor. C3H//HeJ mice became sensitive to each of these effects after adoptive transfer of bone marrow cells from C3H/HeN mice. The efficacy of transfer was directly proportional to the dose of X-irradiation and inversely proportional to the number of surviving host stem cells. The most effective dose of radiation was 850 rad, and C3H/HeN leads to C3H/HeJx chimeras prepared at this dose were as sensitive to LPS for each parameter tested as were the C3H/HeN donors except for a threefold greater resistance to lethality than LPS-responsive C3H/HeN mice. C3H/HeN mice could also be rendered unresponsive to LPS by the adoptive transfer of C3H/HeJ bone marrow cells. C3H/HeN chimeras were resistant to all of the effects of LPS studied except for the induction of colony-stimulating factor. These results demonstrate that lymphocytes and/or macrophages play a primary role in mediating a number of diverse and seemingly unrelated host responses to endotoxin.

NIH Conference: Immunoglobulin G Fc receptor-mediated clearance in autoimmune diseases

The reticuloendothelial system is thought to play an important role in removing immune complexes and other immunologically active substances from the circulation via interaction with specific cell-surface receptors. The function of the reticuloendothelial system in humans with autoimmune diseases was studied in vivo by measuring the rate of removal of IgG-coated, radio-labeled autologous erythrocytes. Such cells are removed by phagocytic cells of the spleen, and the process depends on the presence of an intact IgG Fc fragment. Studies in patients with active systemic lupus erythematosus show a profound defect in Fc-receptor-specific clearance that correlates with disease activity. Patients with other autoimmune diseases have defects in Fc receptor functional activity when their illness is characterized by tissue deposition of immune complexes. Normal patients with HLA-B8/DRw3, an HLA type associated with an increased incidence of autoimmune disease, also have an increased incidence of defective Fc receptor-specific functional activity, suggesting that this defect may predispose patients with this haplotype to develop manifestations of immune complex-mediated disease.

Inherited CARD9 Deficiency: Invasive Disease Caused by Ascomycete Fungi in Previously Healthy Children and Adults

Autosomal recessive CARD9 deficiency underlies life-threatening, invasive fungal infections in otherwise healthy individuals normally resistant to other infectious agents. In less than 10 years, 58 patients from 39 kindreds have been reported in 14 countries from four continents. The patients are homozygous (n = 49; 31 kindreds) or compound heterozygous (n = 9; 8 kindreds) for 22 different CARD9 mutations. Six mutations are recurrent, probably due to founder effects. Paradoxically, none of the mutant alleles has been experimentally demonstrated to be loss-of-function. CARD9 is expressed principally in myeloid cells, downstream from C-type lectin receptors that can recognize fungal components. Patients with CARD9 deficiency present impaired cytokine and chemokine production by macrophages, dendritic cells, and peripheral blood mononuclear cells and defective killing of some fungi by neutrophils in vitro. Neutrophil recruitment to sites of infection is impaired in vivo. The proportion of Th17 cells is low in most, but not all, patients tested. Up to 52 patients suffering from invasive fungal diseases (IFD) have been reported, with ages at onset of 3.5 to 52 years. Twenty of these patients also displayed superficial fungal infections. Six patients had only mucocutaneous candidiasis or superficial dermatophytosis at their last follow-up visit, at the age of 19 to 50 years. Remarkably, for 50 of the 52 patients with IFD, a single fungus was involved; only two patients had IFDs due to two different fungi. IFD recurred in 44 of 45 patients who responded to treatment, and a different fungal infection occurred in the remaining patient. Ten patients died from IFD, between the ages of 12 and 39 years, whereas another patient died at the age of 91 years, from an unrelated cause. At the most recent scheduled follow-up visit, 81% of the patients were still alive and aged from 6.5 to 75 years. Strikingly, all the causal fungi belonged to the phylum Ascomycota: commensal Candida and saprophytic Trychophyton, Aspergillus, Phialophora, Exophiala, Corynesprora, Aureobasidium, and Ochroconis. Human CARD9 is essential for protective systemic immunity to a subset of fungi from this phylum but seems to be otherwise redundant. Previously healthy patients with unexplained invasive fungal infection, at any age, should be tested for inherited CARD9 deficiency.

KEY POINTS

• Inherited CARD9 deficiency (OMIM #212050) is an AR PID due to mutations that may be present in a homozygous or compound heterozygous state. • CARD9 is expressed principally in myeloid cells and transduces signals downstream from CLR activation by fungal ligands. • Endogenous mutant CARD9 levels differ between alleles (from full-length normal protein to an absence of normal protein). • The functional impacts of CARD9 mutations involve impaired cytokine production in response to fungal ligands, impaired neutrophil killing and/or recruitment to infection sites, and defects of Th17 immunity. • The key clinical manifestations in patients are fungal infections, including CMC, invasive (in the CNS in particular) Candida infections, extensive/deep dermatophytosis, subcutaneous and invasive phaeohyphomycosis, and extrapulmonary aspergillosis. • The clinical penetrance of CARD9 deficiency is complete, but penetrance is incomplete for each of the fungi concerned. • Age at onset is highly heterogeneous, ranging from childhood to adulthood for the same fungal disease. • All patients with unexplained IFD should be tested for CARD9 mutations. Familial screening and genetic counseling should be proposed. • The treatment of patients with CARD9 mutations is empirical and based on antifungal therapies and the surgical removal of fungal masses. Patients with persistent/relapsing Candida infections of the CNS could be considered for adjuvant GM-CSF/G-CSF therapy. The potential value of HSCT for CARD9-deficient patients remains unclear.

Complex Roles of Microglial Cells in Ischemic Stroke Pathobiology: New Insights and Future Directions

Ischemic stroke constitutes the major cause of death and disability in the industrialized world. The interest in microglia arose from the evidence outlining the role of neuroinflammation in ischemic stroke pathobiology. Microglia constitute the powerhouse of innate immunity in the brain. Microglial cells are highly ramified, and use these ramifications as sentinels to detect changes in brain homeostasis. Once a danger signal is recognized, cells become activated and mount specialized responses that range from eliminating cell debris to secreting inflammatory signals and trophic factors. Originally, it was suggested that microglia play essentially a detrimental role in ischemic stroke. However, recent reports are providing evidence that the role of these cells is more complex than what was originally thought. Although these cells play detrimental role in the acute phase, they are required for tissue regeneration in the post-acute phases. This complex role of microglia in ischemic stroke pathobiology constitutes a major challenge for the development of efficient immunomodulatory therapies. This review aims at providing an overview regarding the role of resident microglia and peripherally recruited macrophages in ischemic pathobiology. Furthermore, the review will highlight future directions towards the development of novel fine-tuning immunomodulatory therapeutic interventions.