Underappreciated layers of antibody-mediated immune synapse architecture and dynamics

Abstract

The biologic activities of antibody drugs are dictated by structure-function relationships-emerging from the kind, composition, and degree of interactions with a target antigen and with soluble and cellular antibody receptors of the innate immune system. These activities are canonically understood to be both modular: antigen recognition is driven by the heterodimeric antigen-binding fragment, and innate immune recruitment by the homodimeric constant/crystallizable fragment. The model that treats these domains with a high degree of independence has served the field well but is not without limitations. Here, we consider how new insights, particularly from structural studies, complicate the model of neat biophysical separation between these domains and shape our understanding of antibody effector functions. The emerging model endeavors to explain the phenotypic impact of both antibody intrinsic characteristics and extrinsic features-fitting them within a spatiotemporal paradigm that better accounts for observed antibody activities. In this review, we will use insights from recent models of classical complement complexes and T cell immune synapse formation to explore how structural differences in antibody-mediated immune synapses may relate to their functional diversity.

Keywords: antibody; effector function; immune synapse; mechanism of action.

Fig 1

Determinants of antibody immune synapse architecture and dynamics. A broad range of effector, antibody, and target characteristics underlie antibody functional profiles. Antibodies can mediate a diverse set of immunologically and pathologically relevant innate effector functions. The kind(s) and degree to which an antibody elicits these activities is dependent on a number of factors. Given antibody recognition of its target, factors that immediately constrain these activities are the identity, frequency, and status of effectors present in the local tissue, and the biophysical characteristics of the antibody molecule. The former is shaped by host genetics and disease status shaping the microenvironmental milieu. These factors influence the types and expression levels of FcRs on recruited effector cells. Given fully optimal 1-to-1 interactions between a monoclonal antibody and its target antigen, the cell-surface densities of each and their ability to diffuse laterally within the membrane are critical components of signaling and activation. Such factors exist on a continuum and explain some but not all of the functional heterogeneity observed within groups of comparable antibodies. ADCP, antibody-dependent cellular phagocytosis; ADNP, antibody-dependent neutrophil phagocytosis; ADCC, antibody-dependent cellular cytotoxicity; CDR, complementary determining region; FcR, Fc receptor.

Fig 2

T cell immune synapse composition and signaling. The classical T cell immune synapse with central and peripheral regions and molecules that sort by size is pictured at the left and compared with the synapse structure observed for CAR T cells at the right. Signal cascades, shown in the center, differ in association with differences in the spatial organization of the synapse. Created with Biorender.

Fig 3

Structural determination of the classical complement activation complex provides mechanistic framework. (a) The repeating hexagonal lattice structure evident in crystals of an early structure of a whole IgG1 molecule (78) provided evidence that antibodies might self-associate. Diebolder and colleagues leveraged this clue to experimentally demonstrate that antibody-mediated complement activation is a function of (b) Fc:Fc association, in addition to the ability of gC1q to bind to the CH2–hinge region (25). (c) The 3D reconstruction of soluble C1–IgG16 complex (right panel; EMD-4232) as determined using single particle cryo-electron microscopy (80). The left panel illustrates a model of the C1–IgG16 complex with the IgG Fc:gC1q structure (PDB 6FCZ) fit sixfold into the density C1–IgG16 tomograph. (d) Monovalent target engagement, in some cases, promotes more efficient C1q recruitment through both gC1q:Fc recognition site accessibility and stabilization by noncovalent interactions between unliganded Fab and gC1q. gC1q Globular head of the C1q molecule.

Fig 4

Spatial factors at antibody-mediated synapses may partially explain phenotypic heterogeneity observed within groups of similar antibodies. Top. Spatial consequences emerging from the interplay between antibody, effector, and target features at the antibody-mediated immune synapse influence the ability or degree to which cross-linking and activating/inhibitory signaling occurs. Center. Epitope location constrains ability of Fc and other Fab arm(s) to achieve FcR and avid binding, respectively. The asymmetric nature of the Fc makes it possible for binding by either Fab to result in distinct Fc orientation. Bottom. The epitope-paratope structure unique to every antibody-antigen pair, and malleable with antigen conformation or integrity, determines both the steric availability of the Fc and the ability for multivalent modes of binding. Antibody characteristics influencing the overall conformational flexibility affects the probability of antigen cross-linking and FcR interactions. Antigen binding-induced conformational changes may also play a role in regulating antibody-mediated effector functions, as does the flexibility and length of the antibody hinge.