Dendritic cell microvilli: a novel membrane structure associated with the multifocal synapse and T-cell clustering

Abstract

Polarizing effects of productive dendritic cell (DC)-T-cell interactions on DC cytoskeleton have been known in some detail, but the effects on DC membrane have been studied to a lesser extent. We found that T-cell incubation led to DC elongation and segregation of characteristic DC veils to the broader pole of the cell. On the opposite DC pole, we observed a novel membrane feature in the form of bundled microvilli. Each villus was approximately 100 nm in diameter and 600 to 1200 nm long. Microvilli exhibited high density of antigen-presenting molecules and costimulatory molecules and provided the physical basis for the multifocal immune synapse we observed during human DC and T-cell interactions. T cells preferentially bound to this site in clusters often contained both CD4 ؉ and CD8 ؉ T cells. (Blood. 2008;112:5037-5045) IntroductionDendritic cells (DCs) are sentinels of immunity as they respond to environmental cues resulting in antigen uptake and processing and stimulation of antigen-specific T cells. 1,2 In addition, DCs activate the cells of innate immunity, eg, natural killer (NK) cells, NK T cells, and neutrophils. 3 Langerhans cells, the DCs predominantly located in the skin, were originally discovered because of their unique morphology of large neuron-like dendrites. These cells were recognized for their myeloid origin, role in lymphocyte activation, 4-8 and characteristic translucent membrane veils. 5,9 Similar to Langerhans cells in situ, DCs matured from monocytes in vitro display analogous membrane veils. 5,9 T cells are activated through cell-cell contact and by soluble molecules. Early response to cell-cell contact includes the formation of a large macromolecular membrane assembly: the immunologic synapse 10,11 and the consequent cell polarization. 12,13 The initial idea that immunologic synapses mediate T-cell activation has been challenged by the finding that T-cell receptor (TCR)-mediated signaling precedes or is independent of synapse formation. [14][15][16][17] Consequently, the full function of the synapse has been modified to include directed cytokine secretion, enhanced secondary signals, or internalization of TCRs. [18][19][20] Most studies of the synapse have focused on those formed between T cells and B cells [21][22][23][24][25] ; consequently, the synapse between T cells and DCs has been studied to a much lesser extent. In contrast to the unifocal T-cell-B-cell synapse, the structure of the DC-T-cell interaction site, studied mostly in murine cells, appears "immature" or "multifocal," containing numerous segregated microdomains. 26 Microdomain segregation could be important for the physical separation of ligands and receptors in the membrane plane; such separation appears essential in preventing the cis-acting inhibition, such as that observed within the Notch family ligands and receptors during DC-T-cell interaction. 27 In addition, recent evidence suggests that mechanical TCR entrapment into segregated microdomains of the synapse is required for optimal T-cell activation. 28 Even th...

Microvilli set the stage for T-cell activation

“…As the TCR clusters were observed before T-cell activation and were microvilli dependent, these results suggest that microvilli serve as the structural scaffold for clustering of TCRs and for the recruitment of microclusters upon TCR engagement. Moreover, these results are consistent with the observation of preformed TCR clusters in unactivated T cells (8,9) and mirror the selective localization of MHC molecules at the tips of membrane protrusions in APCs (11). This novel role of microvilli in positioning preformed TCR clusters has multiple implications regarding the spatial and dynamic regulation of T-cell activation.…”

High-resolution imaging of the immunological synapse and T-cell receptor microclustering through microfabricated substrates

“…Interestingly, the role of the APC cytoskeleton in the modulation of microcluster dynamics and T-cell activation is reportedly absent in the most highly studied model for IS formation, namely the synaptic interactions between T-cells and antigen-presenting B-cells [10,[44][45][46]. Studies with B-lymphoma APCs have identified the rearrangement of surface receptors on the APC as a passive event and that B-cell cytoskeletal disruption does not modulate T-cell activation [47], however, studies are emerging on the importance of the APC cytoskeleton in IS formation, particularly in DC-mediated T-cell activation [37,42]. Here, the K562 aAPC, being of myeloid origin may share homologies with both DCs and osteoclast cells, also of myeloid lineage [48].…”

Three-dimensional localization of T-cell receptors in relation to microvilli using a combination of superresolution microscopies

“…Interestingly, several EM studies reported well-maintained 3D structures of microvilli at T-cell-APC (26,27) or T-cell-T-cell interfaces (28) in vitro. Nevertheless, the presence of these structures in T cells migrating inside lymph nodes in search for antigen in vivo has been disputed (29)(30)(31)(32)(33).…”

“…Furthermore, our findings raise the intriguing possibility that, similar to L-selectin, whose localization on microvilli is critical for leukocyte capture by L-selectin ligands (6), enrichment of TCRs on these cellular projections may increase the probability of their encounter of antigens presented on counterprojections on APCs, primarily dendritic cells. Indeed, most APCs contain microvilli (26,48,49), and experiments suggested that MHC molecules, on which TCR antigenic ligands are presented, are enriched on these projections (49). Thus, molecules enriched on T-cell and APC microvilli may serve as firsthand participants in the T-cell activation process.…”