The surfaceome architecture of a human B cell

Cellular function is orchestrated by extracellular signaling cues to elicit intracellular responses to stimuli. Cell surface residing proteins, collectively termed the surfaceome, fulfill their function as signal transducers within protein communities. This cell surface receptor interaction network is complex and dynamic, forming a highly organized surfaceome architecture essential for cellular functionality. Exploring the lateral proximity network of cell surface residing proteins facilitates the understanding of the surfaceome architecture, hence allowing for manipulation by utilizing bivalent therapeutics, such as bispecific antibodies. Targeting cells based on specific combinations of proteins present on their surface holds great potential in the search for selecting disease-driving sub-populations. We set out to identify novel target combinations for such therapies by elucidating the surfaceome architecture of B cells. To probe the spatial surfaceome, we utilized the in-house developed technology LUX-MS, enabling light-mediated proximity detection of dynamic protein interactions on the cell surface. Small molecule Singlet Oxygen Generators (SOGs) facilitate light-mediated photo-oxidation of cell surface proteins in the nanometer vicinity. When delivered through a ligand-of-interest, SOGs enable the capture of receptor proximity networks. LUX-MS of the mature B cell marker IgM revealed proximity candidates, of which 15 were initially chosen for individual LUX-MS experiments to confirm their proximity to IgM in a reciprocal approach. Some of these candidates exhibit similar proximity networks to IgM, such as the co-receptor CD79a, but others display very distinct organizations, as in the case of the costimulatory molecule and CD27 ligand CD70. The extensive proximity network of CD70 suggests a bystander functionality due to its high expression levels but can also hint at distinct organizational hubs on a single cell level. Intersecting and distinct proximity networks of individual target receptors facilitate the establishment of a global view of the B cell surfaceome architecture through reciprocal network walking. Having established initial architectural insight into the B cell surfaceome landscape, we are currently expanding our datasets with a large set of CD-annotated receptor targets in an effort to generate a comprehensive view of the B cell surfaceome architecture. Establishing a model for the B cell surfaceome architecture offers an opportunity to expand the toolbox for improved targeted immunotherapy by revealing drug candidate pairs.