
==== Front
Biomed J
Biomed J
Biomedical Journal
2319-4170
2320-2890
Chang Gung University

S2319-4170(23)00067-7
10.1016/j.bj.2023.100630
100630
VSI: Extracellular Vesicles and Exosomes (Review Article)
Physiological and therapeutic relevance of T cell receptor-mediated antigen trogocytosis
Martinez-Martin Nuria
Alarcon Balbino balarcon@cbm.csic.es
∗
Centro de Biología Molecular Severo Ochoa, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain
∗ Corresponding author. Centro de Biología Molecular Severo Ochoa, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Nicolás Cabrera, 1, 28049, Spain. balarcon@cbm.csic.es
15 7 2023
10 2024
15 7 2023
47 5 10063022 5 2023
12 7 2023
© 2023 The Authors
2023
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Trogocytosis is an active process whereby fragments of plasma membrane proteins and cytoplasm are transferred from one cell to another in a cell-cell contact-dependent manner. T cells trogocytose pieces of the cells presenting antigen to them at the site of the immunological synapse. Fragments of the antigen-presenting cell membrane rich in antigen/major histocompatibility (MHC) complexes are internalized by the T cell. Those complexes are redirected to the plasma membrane of the T cell, which subsequently becomes an antigen-presenting cell to other T cells. Removing antigen/MHC complexes from professional and tumoral cells has consequences for the intensity and duration of the immune response. However, the acquired capacity of T cells to present the trogocytosed cognate antigen/MHC complexes also affects the properties of the trogocytotic T cells. Acting as antigen-presenting cells, trogocytotic CD4 T cells influence both the differentiation of cytotoxic T cells and the differentiation of other CD4 T cells into pro-inflammatory effector T cells. Furthermore, trogocytosis of antigen/MHC complexes promotes the differentiation of the trogocytotic CD4 T cells towards regulatory T cells and Th2 effector cells. Trogoctyosis is, therefore, a parallel mechanism to signal transduction by membrane receptors, including the T cell antigen receptor, at the plane of the plasma membrane.

Keywords

T cell receptor
Trogocytosis
MHC
Antigen presentation
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pmcIntroduction

What is trogocytosis, cell types

Trogocytosis (from the Greek Trogos: to gnaw) is the transfer of plasma membrane fragments and cytoplasm from one cell to another in a cell–cell contact-dependent manner. Although not restricted to cells of multicellular organisms or cells of hematopoietic origin, this process has been most extensively studied in cells of the immune system [1]. Cells of both myeloid (macrophages, dendritic cells, neutrophils) and lymphoid (αβ and γδ T cells, NK cells, and B cells) lineages have been shown to trogocytose fragments from other cells. T cell-mediated trogocytosis was first demonstrated by the acquisition and display of allogeneic MHC class II by adoptively transferred murine T cells. Since murine T cells do not express endogenous MHC class II molecules, the only possible source of allogeneic MHC class II expression was the cells and tissues of the recipient mice. The transfer of plasma membrane receptors to trogocytic T cells involves MHC molecules and other molecules typically expressed by different cell lineages, such as membrane IgM of B cell origin.

Main text

Mechanisms of trogocytosis

It has become increasingly clear that the mechanism of trogocytosis is cell type-specific. For example, antigen uptake by B cells was shown to be relatively insensitive to temperature. In contrast, T-cell antigen uptake was strongly inhibited by incubation at 4 °C [2]. Furthermore, various inhibitors of actin polymerization and kinases involved in intracellular antigen receptor signaling inhibited trogocytosis of antigens by T cells but not by B cells. Thus, it appears that, at least for T cells, acquiring membrane fragments containing antigen/MHC complexes from APCs trogocytosis is an active process requiring TCR signaling and actin cytoskeleton rearrangement. The acquisition of antigen/MHC complexes and ligands for costimulatory receptors occurs at the immunological synapse. It has been shown that the cSMAC, the site of higher accumulation of bound TCRs and antigen/MHC on the opposite APC side of the immunological synapse, is the site where the T cell plasma membrane is invaginated and a fragment of the APC membrane and cytoplasm is gnawed [Fig. 1] [11]. The process of TCR-mediated trogocytosis is reminiscent of phagocytosis. It requires zippering of the T cell and APC membranes mediated by TCR activation of actin cytoskeleton rearrangement. For this reason, trogocytosis has been proposed as frustrated phagocytosis, and indeed, cells such as B and T cells that can perform trogocytosis also possess phagocytic capacity [3,4]. Whether the two mechanisms are the same is currently under debate. The RAS family GTPase R-RAS2, which is directly recruited to the TCR, and the Rho family GTPase RhoG, which has previously been implicated in the phagocytosis of apoptotic cells [5,6], are mediators of TCR-triggered trogocytosis of APC membrane fragments [3].Fig. 1 Cartoon and sequence of events that convert a trogocytic T cell into an APC. T cells recognize antigen/MHC complexes on the plasma membrane of a professional APC. This leads to an accumulation of TCR-antigen/MHC complexes at the center of the immunological synapse. Such accumulation triggers polymerization of the actin cytoskeleton by a mechanism involving the GTPases R-RAS2, which activates PI3K, and RhoG. Such a rearrangement of the actin cytoskeleton under the engaged TCRs in the immunological synapse “pulls” the APC membrane, resulting in an invagination of the T-cell membrane that eventually leads to the removal of a fragment of the APC plasma membrane and part of the cytoplasm. The trogocytic vesicle fuses with intracellular endocytic compartments, forming multivesicular bodies (MVBs). In these MVBs, trogocytosed vesicles can either remain as such or fuse with the outer membrane of the MVB. The unfused vesicles of APC origin are released as such when the MVB releases its contents into the extracellular space, forming plasma membrane-associated exosomes rich in acquired antigen/MHC complexes and other receptors taken from the APC. The vesicles fused intracellularly with the outer MVB membrane will be responsible for the presence of trogocytosis-derived antigen/MHC complexes directly on the plasma membrane of the T cell by two membrane fusion events. The trogocytic T cell becomes an antigen-presenting cell for other T cells of the same antigen specificity.

Fig. 1

It is important to note that, in addition to the TCR, other receptors and membrane components mediate and modulate trogocytosis in T cells. Interestingly, this again appears to be cell type-specific. This is the case for regulatory CD4 T cells (Treg), which have been shown to display CTLA4 [5] and CD137 [6] mediated trogocytosis, or PD-1+Tim3+CD8+ tumor-infiltrating lymphocytes, which have been described to display trogocytosis in which the interaction between phosphatidylserine and Tim-3 appears to be critical [7]. These different trogocytotic processes will affect the immune response, as discussed later.

Intracellular signaling during trogocytosis

A burning question is whether the receptor–ligand complexes continue to participate in signal transduction after trogocytosis or stop these signals when the trogocytosed membrane fragments enter intracellular compartments (for a recent review [8]). In CD30+ epithelial cells, it has been shown that CD30 mediates the uptake of CD30L from other cells by trogocytosis and that the internalized CD30-CD30L complexes continue to signal from internal signalosomes [9]. Regarding antigen/MHC uptake by TCR-triggered trogocytosis, it has been generally assumed that TCR internalization arrests TCR signaling. However, recent evidence supports the hypothesis that the internalized TCR continues to signal from specialized endosomes [10]. This idea that the internalized TCR continues to transduce activating signals is consistent with the finding that trogocytic T cells continue to maintain high levels of NFκB and AP-1 activation and produce a specific set of cytokines (IL2, IL9, and IFNγ) after removal of APCs [11]. More direct evidence for TCR signaling from internal compartments has been provided by showing that trogocytosed antigen/MHC II complexes remain bound to the TCR in trogocytic cells and that the TCR continues to give signals, including those elicited by the TCR-proximal ZAP70 tyrosine kinase as well as ERK phosphorylation [12].

Another question is the consequences of this ongoing intracellular signaling by TCR bound to trogocytosed antigen/MHC complexes. It has been shown that trogocytosis-mediated signaling induces both naive and Th1 CD4 T cells to differentiate into Th2 cells [13]. In addition, it has been demonstrated that trogocytotic CD4 T cells continue to express MHC-II for at least 5 days and survive longer than non-trogocytotic T cells, although without undergoing cell division [12]. These results suggest that the TCR remains engaged with its cognate antigen/MHC complexes and signals during at least part of the trogocytic process. Such signaling contributes to the T cell's differentiation and survival program adoption.

Acquisition by trogocytotic T cells of antigen presentation properties

Another aspect of TCR-mediated trogocytosis of APC membrane fragments, including antigen/MHC complexes, is whether trogocytotic T cells display such complexes on their plasma membrane and become antigen-presenting cells for other T cells. Early publications highlighted the negative effect of antigen/MHC uptake by T cells, resulting in the removal of these complexes from APCs, thus limiting clonal expansion induced by alloantigens following both tissue and stem cell transplantation [14]. However, it has also been shown that human allogeneic T cells acquire both HLA-DR and the costimulatory CD80 and CD86 ligands of CD28 and present them to stimulate other T cells, thereby enhancing the response to alloantigens [15]. In contrast, another study showed that naive CD4 T cells capture their cognate antigen/MHC complex from APCs and that its presentation to either other antigen-experienced CD4 T cells or naive CD4 T cells has different consequences, resulting in the prevention of antigen-experienced T cell participation in the response [16]. Interestingly, another study using CD4 T cells from TCR transgenic mice has shown that trogocytotic T cells present antigen/MHC complexes to cognate naïve CD4 T cells, resulting in an entirely divergent differentiation of antigen-presenting trogocytotic T cells into Treg and responding naïve CD4 T cells into Th17. Furthermore, such divergent differentiation has been shown to affect central nervous system inflammation in experimental autoimmune encephalitis (EAE) models and the response to viral infection [17].

Trogocytosis by T cells and its effect on the modulation of the immune response

Based on the above, trogocytosis implies internalizing functional and unmodified proteins from donor cells: MHC, costimulatory molecules, adhesion molecule receptors, tumor antigens, and antigenic pathogens [18]. Notably, the trogocytic capacity and output are cell dependent. This process shapes the immune response by acting on both the trogocytotic and the antigen-presenting cells (For a recent review [19]).

After trogocytosis, it has been shown that a trogocytosis-mediated signaling is initiated in the trogocytotic cell. This signaling can regulate the T cell proliferative signal and maintain cell activation without APC [20,21]. This has been shown to be essential for maintaining the activation of memory T cells [21]. Such signaling has also been shown to shape T cell differentiation by inducing the expression of Th2, Tfh, and Treg-associated effector cytokines and transcription factors [2,18,22]. Trogocytosis can also affect T cell function; this is the case for Treg, in which the acquisition of MHC makes them more immunosuppressive to CD8 T cells [23]. Conversely, trogocytosis can induce fratricidal killing of trogocytotic T cells that have acquired the cognate antigen/MHC complexes [9]. In this context, it has been shown that trogocytosis of target molecules by CAR-expressing T cells leads to tumor-associated antigen loss and the fratricidal killing of CAR T cells, both favoring tumor escape [20,24].

Trogocytotic T cells also acquire new functions, such as an APC, through the MHC context, but also other membrane proteins that can be costimulatory, such as CD28, CD54, CD80 [Fig. 2] or inhibitory molecules, such as PD-L1 [25]. It has been shown that trogocytosis by Treg makes them more regulatory, but also that trogocytosis by effector CD4 T cells makes them more efficient [25]. In this context, it has been shown in a model of influenza infection in vivo that trogocytosis-mediated acquisition of MHC-II by CD8 T cells is essential for recall responses to reinfection [26]. In addition, it has been proposed that the acquisition by CD4 T cells of antigen/MHC-I complexes and costimulatory molecules from DCs may be a mechanism that mediates CD8 T cells to be licensed to kill. In other words, the presentation of cognate antigen/MHC-I complexes to CD8 T cells by trogocytotic CD4 T cells could be the mechanism by which the latter promotes the differentiation and maturation of CD8 T cells into CTLs [22].Fig. 2 Trogocytic CD4 T cells acquire and display cognate MHC-II complexes together with CD28 ligands on their own plasma membrane. Expression of acquired I-Ek CD86 on the plasma membrane of AND CD4+ T cells after 1-h incubation with moth cytochrome c peptide 88–103 (MCCp)-loaded bone marrow-derived dendritic cells, analyzed by ELYRA super-resolution microscopy (a z-axis projection of confocal sections).

Fig. 2

Trogocytosis by T cells has a physical counterpart on professional APCs. Depletion of antigen/MHC complexes and costimulatory receptor ligands from APCs [6] could limit their antigen presentation capacity and dampen the immune response. It also compromises their membrane integrity, leading to target cell death [23]. This makes T-cell trogocytosis a critical process in immune defense against pathogens, infection, and tumorigenesis [18]. In particular, Tregs have been shown to deplete CD80 and CD86 ligands of costimulatory receptors through CTLA4-mediated trogocytosis, thereby reducing the ability of professional APCs to activate other T cells [18]. In addition to CTLA4, Tregs appear to use CD137 to take up the immunostimulatory CD137-L from professional APCs by trogocytosis, rendering them less competent to activate other T cells [21]. Notably, the effect of trogocytosis on the antigen-presenting cell can also have negative consequences regarding the effectiveness of the immune response [18]. Trogocytosis can help infected and tumor cells escape or even become a vector for spreading pathogens [20,24]. A summary of the effects of trogocytosis on the immune response is outlined in [Fig. 3].Fig. 3 Consequences of acquisition of antigen/MHC complexes by trogocytic T cells. Removal of antigen/MHC complexes from professional APCs by trogocytic T cells may render the APCs less effective in subsequent rounds of activation of other T cells of the same antigen specificity. If the APC was a cancer cell, removal of antigen/MHC by trogocytic T cells could render cancer cells less sensitive to attack by cytotoxic T cells. Expressing antigen/MHC by trogocytic T cells may make them susceptible to killing by cytotoxic T cells of the same antigen specificity. Trogocytosis by T cells can increase their survival in the absence of proliferation and influence asymmetric cell division. Trogocytosis of antigen/MHC can increase the activity of Tregs. They can favor the differentiation of trogocytic T cells into Th2 and Tfh while favoring the differentiation of cognate naive CD4 T cells into Th17 when an antigen is presented by a trogocytic T cell. Finally, trogocytic CD4 T cells can present acquired antigen/MHC-I complexes to cognate CD8 T cells, thereby licensing the latter to kill.

Fig. 3

Conclusions

Since trogocytosis was first unmasked in eukaryotic cells, and more precisely in immune cells, several studies have proved its impact on shaping the immune response. The reason is apparent: immune cells acquiring functional molecules not codified in their genome must change their phenotype and function during the puzzling response against external threats. In the case of T cells, trogocytosis was first described as uniquely TCR dependent; however, the discovery of other receptors mediating such process and impacting as much as their TCR counterpart in the T cell function has widely opened a new area of study. It will be worth investigating new trogocytosis receptors to understand which stages of the immune response are modulated by trogocytosis.

Interestingly, it is also clear that not only are membrane proteins interchanged, but cytoplasmic content is also transferred. The nature of this cytoplasmic content, how it impinges on trogocytic cells, and whether cellular structures, including organelles, can be interchanged in these processes are worth studying. Moreover, processes called until now as trans-infection or trans-phagocytosis could be specific types of trogocytosis, and molecular mechanisms involved in TCR-trogocytosis could also play a role in these events.

Overall, trogocytosis seems not only cell-dependent but also it appears to be receptor-dependent which could be triggered under different circumstances. Describing these peculiarities should help understand trogocytosis's role upon antigen encounter and develop new therapeutic strategies to improve, among others, immunotherapy protocols.

Conflict of interest statement

The authors declare no conflicts of interests.

Dr. Nuria Martinez

Prof. Balbino Alarcon

Acknowledgments

This work was supported by grant PID2019-104935RB-I00 from the 10.13039/501100004837 Ministry of Science and Innovation of Spain .

Peer review under responsibility of Chang Gung University.
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