Kristopher D. Marjon

Structural recognition and functional activation of FcγR by innate pentraxins

Pentraxins are a family of ancient innate immune mediators conserved throughout evolution. The classical pentraxins include serum amyloid P component (SAP) and C-reactive protein, which are two of the acute-phase proteins synthesized in response to infection. Both recognize microbial pathogens and activate the classical complement pathway through C1q (refs 3 and 4). More recently, members of the pentraxin family were found to interact with cell-surface Fcγ receptors (FcγR) and activate leukocyte-mediated phagocytosis. Here we describe the structural mechanism for pentraxin’s binding to FcγR and its functional activation of FcγR-mediated phagocytosis and cytokine secretion. The complex structure between human SAP and FcγRIIa reveals a diagonally bound receptor on each SAP pentamer with both D1 and D2 domains of the receptor contacting the ridge helices from two SAP subunits. The 1:1 stoichiometry between SAP and FcγRIIa infers the requirement for multivalent pathogen binding for receptor aggregation. Mutational and binding studies show that pentraxins are diverse in their binding specificity for FcγR isoforms but conserved in their recognition structure. The shared binding site for SAP and IgG results in competition for FcγR binding and the inhibition of immune-complex-mediated phagocytosis by soluble pentraxins. These results establish antibody-like functions for pentraxins in the FcγR pathway, suggest an evolutionary overlap between the innate and adaptive immune systems, and have new therapeutic implications for autoimmune diseases.

Pentraxins and Fc receptors

Pentraxins are innate pattern recognition molecules whose major function is to bind microbial pathogens or cellular debris during infection and inflammation and, by doing so, contribute to the clearance of necrotic cells as well as pathogens through complement activations. Fc receptors are the cellular mediators of antibody functions. Although conceptually separated, both pentraxins and antibodies are important factors in controlling acute and chronic inflammation and infections. In recent years, increasing experimental evidence suggests a direct link between the innate pentraxins and humoral Fc receptors. Specifically, both human and mouse pentraxins recognize major forms of Fc receptors in solution and on cell surfaces with affinities similar to antibodies binding to their low affinity Fc receptors. Like immune complex, pentraxin aggregation and opsonization of pathogen result in Fc receptor and macrophage activation. The recently published crystal structure of human serum amyloid P (SAP) in complex with FcγRIIA further illustrated similarities to antibody recognition. These recent findings implicate a much broader role than complement activation for pentraxins in immunity. This review summarizes the structural and functional work that bridge the innate pentraxins and the adaptive Fc receptor functions. In many ways, pentraxins can be regarded as innate antibodies.

Recognition and functional activation of the human IgA receptor (FcαRI) by C-reactive protein

C-reactive protein (CRP) is an important biomarker for inflammatory diseases. However, its role in inflammation beyond complement-mediated pathogen clearance remains poorly defined. We identified the major IgA receptor, FcαRI, as a ligand for pentraxins. CRP recognized FcαRI both in solution and on cells, and the pentraxin binding site on the receptor appears distinct from that recognized by IgA. Further competitive binding and mutational analysis showed that FcαRI bound to the effector face of CRP in a region overlapping with complement C1q and Fcγ receptor (FcγR) binding sites. CRP cross-linking of FcαRI resulted in extracellular signal-regulated kinase (ERK) phosphorylation, cytokine production, and degranulation in FcαRI-transfected RBL cells. In neutrophils, CRP induced FcαRI surface expression, phagocytosis, and TNF-α secretion. The ability of CRP to activate FcαRI defines a function for pentraxins in inflammatory responses involving neutrophils and macrophages. It also highlights the innate aspect of otherwise humoral immunity-associated antibody receptors.

The membrane scaffold CD82 regulates cell adhesion by altering α4 integrin stability and molecular density

CD82 expression alters cell–matrix adhesion and integrin surface expression. The use of superresolution microscopy together with protein clustering algorithms leads to identification of a critical role for CD82 in regulating the molecular density of the α4 integrin.

Macrophages Activated by C-Reactive Protein through FcγRI Transfer Suppression of Immune Thrombocytopenia

C-reactive protein (CRP) is an acute-phase protein with therapeutic activity in mouse models of systemic lupus erythematosus and other inflammatory and autoimmune diseases. To determine the mechanism by which CRP suppresses immune complex disease, an adoptive transfer system was developed in a model of immune thrombocytopenic purpura (ITP). Injection of 200 μg of CRP 24 h before induction of ITP markedly decreased thrombocytopenia induced by anti-CD41. CRP-treated splenocytes also provided protection from ITP in adoptive transfer. Splenocytes from C57BL/6 mice were treated with 200 μg/ml CRP for 30 min, washed, and injected into mice 24 h before induction of ITP. Injection of 106 CRP-treated splenocytes protected mice from thrombocytopenia, as did i.v. Ig-treated but not BSA-treated splenocytes. The suppressive cell induced by CRP was found to be a macrophage by depletion, enrichment, and the use of purified bone marrow-derived macrophages. The induction of protection by CRP-treated cells was dependent on FcRγ-chain and Syk activation, indicating an activating effect of CRP on the donor cell. Suppression of ITP by CRP-treated splenocytes required FcγRI on the donor cell and FcγRIIb in the recipient mice. These findings suggest that CRP generates suppressive macrophages through FcγRI, which then act through an FcγRIIb-dependent pathway in the recipient to decrease platelet clearance. These results provide insight into the mechanism of CRP regulatory activity in autoimmunity and suggest a potential new therapeutic approach to ITP.

Tetraspanin CD82 regulates bone marrow homing of acute myeloid leukemia by modulating the molecular organization of N-cadherin.

Communication between acute myeloid leukemia (AML) and the bone marrow microenvironment is known to control disease progression. Therefore, regulation of AML cell trafficking and adhesion to the bone marrow is of significant interest. In this study, we demonstrate that differential expression of the membrane scaffold CD82 modulates the bone marrow homing of AML cells. By combining mutational analysis and super-resolution imaging, we identify membrane protein clustering by CD82 as a regulator of AML cell adhesion and bone marrow homing. Cluster analysis of super-resolution data indicates that N-linked glycosylation and palmitoylation of CD82 are both critical modifications that control the microdomain organization of CD82 as well as the nanoscale clustering of associated adhesion protein, N-cadherin. We demonstrate that the inhibition of CD82 glycosylation increases the molecular packing of N-cadherin and promotes the bone marrow homing of AML cells. In contrast, we find that the inhibition of CD82 palmitoylation disrupts the formation and organization of N-cadherin clusters and significantly diminishes bone marrow trafficking of AML. Taken together, these data establish a mechanism where the membrane organization of CD82, through specific posttranslational modifications, regulates N-cadherin clustering and membrane density, which impacts the in vivo trafficking of AML cells. As such, these observations provide an alternative model for targeting AML where modulation of protein organization within the membrane may be an effective treatment therapy to disrupt the bone marrow homing potential of AML cells.

Pentraxins and IgA share a binding hot‐spot on FcαRI

The pentraxins, C‐reactive protein (CRP), and serum amyloid P component (SAP) have previously been shown to function as innate opsonins through interactions with Fcγ receptors. The molecular details of these interactions were elucidated by the crystal structure of SAP in complex with FcγRIIA. More recently, pentraxins were shown to bind and activate FcαRI (CD89), the receptor for IgA. Here, we used mutations of the receptor based on a docking model to further examine pentraxin recognition by FcαRI. The solution binding of pentraxins to six FcαRI alanine cluster mutants revealed that mutations Y35A and R82A, on the C‐and F‐strands of the D1 domain, respectively, markedly reduced receptor binding to CRP and SAP. These residues are in the IgA‐binding site of the receptor, and thus, significantly affected receptor binding to IgA. The shared pentraxin and IgA‐binding site on FcαRI is further supported by the results of a solution binding competition assay. In addition to the IgA‐binding site, pentraxins appear to interact with a broader region of the receptor as the mutation in the C′‐strand (R48A/E49A) enhanced pentraxin binding. Unlike Fcγ receptors, the H129A/I130A and R178A mutations on the BC‐ and FG‐loops of D2 domain, respectively, had little effect on FcαRI binding to the pentraxins. In conclusion, our data suggest that the pentraxins recognize a similar site on FcαRI as IgA.

Chronic lymphocytic leukemia with clinical debut as neurological involvement: a rare phenomenon and the need for better predictive markers

BackgroundChronic lymphocytic leukemia (CLL) is the most common leukemia in Western countries. The frequency of symptomatic central nervous system (CNS) involvement is unknown but thought to be a rare phenomenon. Currently there are no known risk factors for CNS involvement.Case presentationWe describe a clinically staged low-risk CLL case that presented with symptomatic CNS involvement and progressed rapidly to death. Evaluation of the surface adhesion molecules identified a markedly altered expression pattern of the integrin, CD49d, and the tetraspanin, CD82, in the index case when compared to similar low-risk CLL cases. We found that the early Rai clinical stage CLL patients showed linear correlation for the co-expression of CD82 and CD49d. In contrast, this unique index case with CNS involvement, which has the same Rai clinical stage, had a significantly lower expression of CD82 and higher expression of CD49d.ConclusionsThese data suggest that the expression profile of CD49d and CD82 may represent potential biomarkers for patients with increased propensity of CNS involvement. Moreover, this study illustrates the critical need for a better mechanistic understanding of how specific adhesion proteins regulate the interactions between CLL cells and various tissue sites.

Measurement of intercellular transfer to signaling endosomes.

Cell-cell communication is essential for an abundance of physiological processes. As such, various mechanisms have evolved to regulate and ensure proper cell-to-cell signaling. Recently, a novel mechanism of cell communication has emerged which involves the physical transfer of proteins, lipids, and nucleic acids between cells. Following this process termed intercellular transfer (ICT), the transferred molecules can signal within recipient cells by entering the endosomal system and trafficking to signaling endosomes. Signaling endosomes can modulate signal localization within the cell as well as the specificity of, and cross talk between, signaling pathways. As such, ICT into signaling endosomes has the potential to modify the signaling profile of the recipient cell. In this chapter, we describe the different methods of ICT as well as how transfer to signaling endosomes can be visualized and quantified.