Macrophages move captured proteins onto their own surface during live-cell uptake

Macrophages, the sentinel cells of the innate immune system, have long been celebrated for their remarkable ability to engulf and process foreign material, from invading bacteria to the cellular debris left behind by dying tissue. A new study published in Nature Chemical Biology adds a surprising twist to this familiar story. The research reports that during live-cell uptake, macrophages do not simply internalize and digest the functional proteins they capture; a subset of these proteins is instead transferred to the macrophage surface, where it remains functional and accessible to the extracellular environment. The finding, described in work published at https://www.nature.com/articles/s41589-026-02292-0, challenges the assumption that engulfment is synonymous with destruction and suggests that the macrophage surface may function as a dynamic display platform shaped by whatever the cell has recently consumed.

The conceptual foundation of the study rests on a tension that immunologists have wrestled with for decades. The classical view of phagocytosis describes a one-way road: a target particle is recognized by receptors on the macrophage membrane, enveloped by actin-driven membrane extension, sealed inside an intracellular vesicle called a phagosome, and then progressively acidified and enzymatically degraded as the phagosome matures through fusion with lysosomes. Under this model, anything the macrophage eats is destined for the degradative pathway. Peptides derived from digested proteins are loaded onto major histocompatibility complex molecules and presented to lymphocytes, closing the loop between innate scavenging and adaptive surveillance. The new work suggests that this pathway is not the only fate available to captured material, and that functional protein transfer to the plasma membrane competes with degradation during uptake.

Technically, the distinction between internalization and surface transfer is not trivial to demonstrate, because material that remains attached to the outside of a cell can masquerade as internalized cargo in conventional flow cytometry and bulk fluorescence assays. Experiments of this kind therefore depend on approaches that spatially resolve the membrane. The study’s conclusions hinge on the ability to distinguish proteins that have genuinely been routed to the macrophage surface from those merely riding on incompletely internalized particles or trapped in membrane ruffles. Proteins delivered to the surface in a functional state must retain at least some of their biochemical activity, a criterion that separates this phenomenon from passive adsorption of denatured fragments. The authors’ characterization of functionally active proteins appearing on the macrophage membrane after uptake thus implies a controlled trafficking event rather than an artifact of sample handling.

One implication of the finding concerns the growing appreciation of trogocytosis, the process by which cells exchange fragments of their plasma membrane and surface molecules through contact. Trogocytosis has been documented most extensively among lymphocytes and antigen-presenting cells, where a cell can literally strip membrane-associated ligands from a partner and wear them on its own surface. The macrophage behavior described in the new study can be understood as a related but distinct phenomenon: rather than acquiring proteins from another cell through direct intermembrane contact during a competitive interaction, the macrophage appears to reroute a portion of the cargo it engulfs back to its own membrane during the uptake process itself. The phrase live-cell uptake in the study’s title is significant, because it indicates that this transfer occurs when the macrophage consumes material from living cells, situations in which the membrane chemistry of the target and the dynamics of receptor engagement differ substantially from uptake of dead cells or inert particles.

The biochemical questions raised by the work are considerable. For a protein to appear on the external face of the macrophage plasma membrane in a functional form, it must traverse or bypass several membrane barriers. Cargo internalized by phagocytosis is enclosed within a vesicle whose lumen is topologically extracellular, which means that, in principle, a protein could reach the cell surface by fusion of recycling vesicles with the plasma membrane without ever entering the cytosol. This recycling route is well established for receptors that are internalized and returned to the surface, and the new study suggests that at least some captured functional proteins can piggyback on analogous recycling traffic. Alternatively, transfer could involve direct membrane continuity between the forming phagosome and the plasma membrane, or regurgitation of incompletely sealed uptake structures. Distinguishing among these routes is a central challenge for follow-up work.

Functional display of captured proteins could have far-reaching consequences for immune regulation. A macrophage that presents an active, intact protein on its surface is not merely advertising peptides for T cell inspection; it is offering other cells the opportunity to bind that protein, respond to its enzymatic activity, engage it as a ligand, or be inhibited by it. If the transferred proteins include, for example, receptors, adhesion molecules, complement regulators, or signaling ligands derived from the cells the macrophage has consumed, the macrophage could effectively adopt surface properties of its prey. Such molecular mimicry at the single-cell level would provide a mechanism by which tissue-resident macrophages continually update their surface identity to reflect the local environment they patrol, blurring the boundary between self-display and scavenged display.

The finding also speaks to long-standing puzzles in the biology of macrophage interactions with living cells. Macrophages routinely sample healthy cells through brief contacts and transient uptake events without triggering inflammation, a process that depends on the balance of activating and inhibitory signals received through receptors such as those in the signal regulatory protein and integrin families. If live-cell uptake can leave functional proteins on the macrophage surface, then even a fleeting phagocytic event could durably alter the macrophage’s signaling landscape. Proteins acquired from a healthy cell might include inhibitory ligands that reinforce tolerance, whereas proteins acquired from a stressed or transformed cell might advertise danger. In this way, surface protein transfer could convert every meal a macrophage takes into a change in its own phenotype, coupling immune surveillance at the level of tissues to reprogramming at the level of the single cell.

From the perspective of chemical biology, the study exemplifies a broader trend of interrogating immune phenomena with tools that track molecules rather than populations. Understanding that captured proteins can remain functional after transfer requires assays that measure activity, localization, and trafficking simultaneously, integrating live-cell imaging, biochemical fractionation of membrane compartments, and perturbation of vesicular transport pathways. The paper’s home in Nature Chemical Biology underscores this methodological character: the question is not only what the macrophage does, but how molecular movement between intracellular compartments and the plasma membrane can be resolved, quantified, and manipulated. Insights of this kind are likely to inform the design of drug delivery systems, because nanoparticles and antibody conjugates engineered for macrophage uptake may likewise find themselves displayed, intact and active, on the macrophage surface rather than sequestered internally.

Therapeutically, the implications span several domains. In cancer immunotherapy, macrophages infiltrating tumors are known to engulf tumor cells and tumor-derived material, and their subsequent behavior profoundly shapes the antitumor response. If live-cell uptake leaves functional tumor proteins on the macrophage surface, this could either help prime adaptive immunity by displaying intact targets for antibody binding, or subvert it by presenting tolerogenic ligands. In infectious disease, pathogens that manipulate phagocytosis might exploit the transfer pathway to decorate macrophages with their own surface molecules, a strategy that could aid immune evasion. In transplantation and autoimmunity, acquired display of donor- or self-derived functional proteins could tilt local immune signaling toward acceptance or attack. Each of these scenarios remains speculative pending direct evidence about which proteins are transferred and under what physiological conditions, but they illustrate why a shift in the fate map of phagocytosed material matters well beyond cell biology.

The study ultimately reframes the macrophage surface as an interface in constant negotiation with the cell’s dietary history. Rather than a fixed identity defined by genome-encoded receptor expression, the macrophage membrane emerges as a composite structure, continuously edited by the functional proteins the cell captures from its surroundings during live-cell uptake. Future work will need to identify the molecular machinery that directs captured proteins to the surface, determine the breadth of cargo that follows this route, establish how long acquired proteins persist and signal, and test whether the phenomenon operates in vivo across tissues and disease states. What the current finding establishes is that the degradative pipeline of phagocytosis has a branch point that earlier models did not anticipate, and that branch point places captured, functional proteins directly in the traffic of the immune system’s most voracious and influential scavenger cells.

Subject of Research: Protein transfer to the macrophage surface during live-cell phagocytic uptake

Article Title: Macrophages transfer functional proteins to their surface during live-cell uptake

Article References: Volk, R. F., Fan, A. C., Casebeer, S. W., Tejus, V. R., Condon, A. C., Zirak, B., Manon, N. A., Irkliyenko, I., Torralba, D. M., Tao, S., Pollini, T., Ramani, V., Maker, A. V., Krummel, M. F., Goodarzi, H., & Zaro, B. W. (2026). Macrophages transfer functional proteins to their surface during live-cell uptake. Nature Chemical Biology. https://doi.org/10.1038/s41589-026-02292-0

Image Credits: AI Generated

DOI: 10.1038/s41589-026-02292-0

Keywords: macrophages, phagocytosis, live-cell uptake, protein transfer, cell surface display, trogocytosis, innate immunity, membrane trafficking, Nature Chemical Biology, immune regulation, phagosome recycling, chemical biology