Nadine Fernandez

Specificity and affinity of human Fcγ receptors and their polymorphic variants for human IgG subclasses

Distinct genes encode 6 human receptors for IgG (hFcgammaRs), 3 of which have 2 or 3 polymorphic variants. The specificity and affinity of individual hFcgammaRs for the 4 human IgG subclasses is unknown. This information is critical for antibody-based immunotherapy which has been increasingly used in the clinics. We investigated the binding of polyclonal and monoclonal IgG1, IgG2, IgG3, and IgG4 to FcgammaRI; FcgammaRIIA, IIB, and IIC; FcgammaRIIIA and IIIB; and all known polymorphic variants. Wild-type and low-fucosylated IgG1 anti-CD20 and anti-RhD mAbs were also examined. We found that (1) IgG1 and IgG3 bind to all hFcgammaRs; (2) IgG2 bind not only to FcgammaRIIA(H131), but also, with a lower affinity, to FcgammaRIIA(R131) and FcgammaRIIIA(V158); (3) IgG4 bind to FcgammaRI, FcgammaRIIA, IIB and IIC and FcgammaRIIIA(V158); and (4) the inhibitory receptor FcgammaRIIB has a lower affinity for IgG1, IgG2, and IgG3 than all other hFcgammaRs. We also identified parameters that determine the specificity and affinity of hFcgammaRs for IgG subclasses. These results document how hFcgammaR specificity and affinity may account for the biological activities of antibodies. They therefore highlight the role of specific hFcgammaRs in the therapeutic and pathogenic effects of antibodies in disease.

NK Cell Responsiveness Is Tuned Commensurate with the Number of Inhibitory Receptors for Self-MHC Class I: The Rheostat Model

Inhibitory receptors that engage self-MHC class I molecules enable NK cells to detect disease-associated loss of MHC class I on surrounding cells. Previous studies showed that some NK cells lack all receptors for self-MHC class I, yet fail to exhibit autoimmunity because they are generally hyporesponsive to stimulation. We asked whether NK cells exist in only two states, responsive and hyporesponsive, corresponding to cells that express or fail to express inhibitory receptors for self-MHC class I. The alternative model is that NK cells vary continuously in their responsiveness, based on variations in the number of different inhibitory and stimulatory receptors they express, which is known to vary. In this study, we show in the murine system that NK cell responsiveness increases quantitatively with each added self-MHC-specific inhibitory receptor. Genetic analysis demonstrated that interactions of each of the receptors with self-MHC class I were necessary to observe augmented responsiveness. These findings suggest that NK cell responsiveness is comparable to a rheostat: it is tuned to an optimal set point depending on the inhibitory and stimulatory interactions encountered in the normal environment, so as to ensure self-tolerance and yet optimize sensitivity to changes in normal cells.

Natural killer cell differentiation driven by Tyro3 receptor tyrosine kinases

Although understanding of the function and specificity of many natural killer (NK) cell receptors is increasing, the molecular mechanisms regulating their expression during late development of NK cells remain unclear. Here we use representational difference analysis to identify molecules required for late NK cell differentiation. Axl protein tyrosine kinase, together with the structurally related receptors Tyro3 and Mer, were essential for NK cell functional maturation and normal expression of inhibitory and activating NK cell receptors. Also, all three receptors were expressed in maturing NK cells, the ligands of these receptors were produced by bone marrow stromal cells, and recombinant versions of these ligands drove NK cell differentiation in vitro. These results collectively suggest that Axl, Tyro3 and Mer transmit signals that are essential for the generation of a functional NK cell repertoire.