Putting out the fire: coordinated suppression of the innate and adaptive immune systems by SOCS1 and SOCS3 proteins.

The mounting of an effective immune response requires the coordinated function of both the innate and the adaptive arm of the immune system. Cells from both types of immunity respond to antigenic stimuli through a variety of surface and intracellular receptors and produce cytokines that tightly orchestrate the inflammatory response. The operation of feedback control mechanisms that regulate the duration and the amplitude of antigenic and cytokine receptor signaling is therefore required to prevent hyper-activation of the immune system that could lead to tissue destruction or autoimmunity. Suppressor of cytokine signaling (SOCS) proteins have been identified as a negative feedback loop to cytokine signaling. Recently, the generation of genetically engineered mouse models permitted the evaluation of their function in different processes of the immune responses. In this article, we review new insights into the modular structure of SOCS proteins and the function of SOCS1 and SOCS3 to negatively regulate activation and/or differentiation pathways in macrophages, dendritic cells, and T lymphocytes. Thus, SOCS family proteins are components of an emerging immunoregulatory mechanism that maintains the coordinated balance of both innate and adaptive immune responses.

The complement regulator C4b-binding protein (C4BP) interacts with both the C4c and C4dg subfragments of the parent C4b ligand: evidence for synergy in C4BP subsite binding.

C4b-binding protein (C4BP) is a multimeric serum protein that is a potent regulator of the classical and lectin complement pathways. The binding site for C4b has been localized to complement control protein (CCP) domains 1-3 of the C4BP alpha-chain and, in particular, to a cluster of positively charged amino acids predicted to be at the interface between CCP 1 and CCP 2. To determine the regions of C4b contributing to C4BP binding, we have examined via surface plasmon resonance technology the binding of the C4c and C4dg subfragments of C4b to C4BP. At half-physiologic ionic strength, specific and saturable binding was observed for both C4c and C4dg. C4c exhibited much greater ionic strength sensitivity in its binding than did C4dg. Analysis of the effect on binding of the subfragments to various C4b-binding-defective C4BP mutants, together with cross-competition experiments, suggests that the subsites in C4BP for C4c and C4dg are adjacent, but distinct. Additionally, we observed synergy in subsite filling such that the presence of C4dg enhanced the extent of C4c binding over its basal level, and vice versa. The enhanced binding of C4c in the presence of C4dg was not due to an increase in affinity but rather reflected a 2-3-fold increase in the number of sites capable of binding C4c. This suggests the existence of a conformational equilibrium between high- and low-affinity states in the C4c binding subsite within each C4BP subunit, an equilibrium which is shifted in favor of the high-affinity state by the filling of the C4dg subsite.

The C4A and C4B isotypic forms of human complement fragment C4b have the same intrinsic affinity for complement receptor 1 (CR1/CD35).

Several previous reports concluded that the C4b fragment of human C4A (C4Ab) binds with higher affinity to CR1 than does C4Bb. Because the isotypic residues, (1101)PCPVLD and (1101)LSPVIH in C4A and C4B, respectively, are located within the C4d region, one may have expected a direct binding contribution of C4d to the interaction with CR1. However, using surface plasmon resonance as our analytical tool, with soluble rCR1 immobilized on the biosensor chip, we failed to detect significant binding of C4d of either isotype. By contrast, binding of C4c was readily detectable. C4A and C4B, purified from plasma lacking one of the isotypes, were Cs converted to C4Ab and C4Bb. Spontaneously formed disulfide-linked dimers were separated from monomers and higher oligomers by sequential chromatographic steps. The binding sensorgrams of C4Ab and C4Bb monomers as analytes reached steady state plateaus, and these equilibrium data yielded essentially superimposable saturation curves that were well fit by a one-site binding model. Although a two-site model was required to fit the equilibrium-binding data for the dimeric forms of C4b, once again there was little difference in the K(D) values obtained for each isotype. Independent verification of our surface plasmon resonance studies came from ELISA-based inhibition experiments in which monomers of C4Ab and C4Bb were equipotent in inhibiting the binding of soluble CR1 to plate-bound C4b. Although divergent from previous reports, our results are consistent with recent C4Ad structural data that raised serious doubts about there being a conformational basis for the previously reported isotypic differences in the C4b-CR1 interaction.

Research on complement: old issues revisited and a novel sphere of influence.

Immunology in recent years has taken a somewhat surprising turn, expressed by a renewed interest in innate immunity. Especially intriguing is the regulatory role exerted by the innate components on the adaptive response, with Toll receptors and complement components being the most investigated. This function has been firmly established for complement protein CR2 (CD21) as part of the BCR co-receptor CD19/CD21/CD81. New findings are now providing a broader picture of complement and its tuning of the immune response; for example, complement proteins have been implicated in the control of T-cell-mediated responses. We will review some of these data here and summarize new discoveries in areas of research on more traditional topics within the complement literature, such as complement and renal diseases, and the therapeutic use of C1-Inhibitor. We cover papers selected from studies presented at the XIX International Complement Workshop, held in Palermo in September 2002, and published within the past six months.

X-ray crystal structure of the C4d fragment of human complement component C4.

C4 fulfills a vital role in the propagation of the classical and lectin pathways of the complement system. Although there are no reports to date of a C4 functional activity that is mediated solely by the C4d region, evidence clearly points to it having a vital role in a number of the properties of native C4 and its major activation fragment, C4b. Contained within the C4d region are the thioester-forming residues, the four isotype-specific residues controlling the C4A/C4B transacylation preferences, a binding site for nascent C3b important in assembling the classical pathway C5 convertase and determinants for the Chido/Rodgers (Ch/Rg) blood group antigens. In view of its functional importance, we undertook to determine the three-dimensional structure of C4d by X-ray crystallography. Here we report the 2.3A resolution structure of C4Ad, the C4d fragment derived from the human C4A isotype. Although the approximately 30% sequence identity between C4Ad and the corresponding fragment of C3 might be expected to establish a general fold similarity between the two molecules, C4Ad in fact displays a fold that is essentially superimposable on the structure of C3d. By contrast, the electrostatic characteristics of the various faces of the C4Ad molecule show marked differences from the corresponding faces of C3d, likely reflecting the differences in function between C3 and C4. Residues previously predicted to form the major Ch/Rg epitopes were proximately located and accessible on the concave surface of C4Ad. In addition to providing further insights on the current models for the covalent binding reaction, the C4Ad structure allows one to rationalize why C4d is not a ligand for complement receptor 2. Finally the structure allows for the visualization of the face of the molecule containing the binding site for C3b utilized in the assembly of classical pathway C5 convertase.