Publications

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CD45 functions as a signaling gatekeeper in T cells

*Courtney, A.H., Shvets, A., Lu, W., Griffante, G., Mollenauer, M., Horkova, V., Lo, W.L., Yu, S., Stepanek, O., Chakraborty A.K., *Weiss, A. 

Science Signaling. 2019, 12, eaaw8151.  

*Corresponding author (F1000 featured article

T cells require the protein tyrosine phosphatase CD45 to detect and respond to antigen because it activates the Src family kinase Lck, which phosphorylates the T cell antigen receptor (TCR) complex. CD45 activates Lck by opposing the negative regulatory kinase Csk. Paradoxically, CD45 has also been implicated in suppressing TCR signaling by dephosphorylating the same signaling motifs within the TCR complex upon which Lck acts. We sought to reconcile these observations using chemical and genetic perturbations of the Csk/CD45 regulatory axis incorporated with computational analyses. Specifically, we titrated the activities of Csk and CD45 and assessed their influence on Lck activation, TCR-associated ζ-chain phosphorylation, and more downstream signaling events. Acute inhibition of Csk revealed that CD45 suppressed ζ-chain phosphorylation and was necessary for a regulatable pool of active Lck, thereby interconnecting the activating and suppressive roles of CD45 that tune antigen discrimination. CD45 suppressed signaling events that were antigen independent or induced by low-affinity antigen but not those initiated by high-affinity antigen. Together, our findings reveal that CD45 acts as a signaling "gatekeeper," enabling graded signaling outputs while filtering weak or spurious signaling events.

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TCR Signaling: Mechanisms of initiation and propagation

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Courtney, A.H., Lo, W.L., Weiss, A.

Trends in Biochemical Sciences. 2018, 43 (2), 108-123. 

(F1000 featured article)

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The mechanisms by which a T cell detects antigen using its T cell antigen receptor (TCR) are crucial to our understanding of immunity and the harnessing of T cells therapeutically. A hallmark of the T cell response is the ability of T cells to quantitatively respond to antigenic ligands derived from pathogens while remaining inert to similar ligands derived from host tissues. Recent studies have revealed exciting properties of the TCR and the behaviors of its signaling effectors that are used to detect and discriminate between antigens. Here we highlight these recent findings, focusing on the proximal TCR signaling molecules Zap70, Lck, and LAT, to provide mechanistic models and insights into the exquisite sensitivity and specificity of the TCR.

A phosphosite within the SH2 domain of Lck regulates its activation by CD45

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Courtney, A.H., Amacher, J.A., Kadlecek, T.A., Mollenauer, M.N., Au-Yeung, B.B., Kuriyan, J., Weiss, A. 

Molecular Cell. 2017, 67 (3), 498-511.

The Src Family kinase Lck sets a critical threshold for T cell activation because it phosphorylates the TCR complex and the Zap70 kinase. How a T cell controls the abundance of active Lck molecules remains poorly understood. We have identified an unappreciated role for a phosphosite, Y192, within the Lck SH2 domain that profoundly affects the amount of active Lck in cells. Notably, mutation of Y192 blocks critical TCR-proximal signaling events and impairs thymocyte development in retrogenic mice. We determined that these defects are caused by hyperphosphorylation of the inhibitory C-terminal tail of Lck. Our findings reveal that modification of Y192 inhibits the ability of CD45 to associate with Lck in cells and dephosphorylate the C-terminal tail of Lck, which prevents its adoption of an active open conformation. These results suggest a negative feedback loop that responds to signaling events that tune active Lck amounts and TCR sensitivity.

Small molecule inhibition of Csk alters affinity recognition by T cells 

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Manz, B.N., Tan, X.Y., Courtney, A.H., Rutaganira, F., Palmer, E., Shokat, K.M., Weiss, A.,

eLife. 2015, 4, e08088. 

The C-terminal Src kinase (Csk), the primary negative regulator of Src-family kinases (SFK), plays a crucial role in controlling basal and inducible receptor signaling. To investigate how Csk activity regulates T cell antigen receptor (TCR) signaling, we utilized a mouse expressing mutated Csk (Csk(AS)) whose catalytic activity is specifically and rapidly inhibited by a small molecule. Inhibition of Csk(AS) during TCR stimulation led to stronger and more prolonged TCR signaling and to increased proliferation. Inhibition of Csk(AS) enhanced activation by weak but strictly cognate agonists. Titration of Csk inhibition revealed that a very small increase in SFK activity was sufficient to potentiate T cell responses to weak agonists. Csk plays an important role, not only in basal signaling, but also in setting the TCR signaling threshold and affinity recognition.

Multivalent antigens for promoting B and T cell activation

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Bennett, N.R., Zwick, D.B., Courtney, A.H., Kiessling, L.L.

ACS Chemical Biology. 2015, 10 (8), 1817-1824.

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Efficacious vaccines require antigens that elicit productive immune system activation. Antigens that afford robust antibody production activate both B and T cells. Elucidating the antigen properties that enhance B-T cell communication is difficult with traditional antigens. We therefore used ring-opening metathesis polymerization to access chemically defined, multivalent antigens containing both B and T cell epitopes to explore how antigen structure impacts B cell and T cell activation and communication. The bifunctional antigens were designed so that the backbone substitution level of each antigenic epitope could be quantified using (19)F NMR. The T cell peptide epitope was appended so that it could be liberated in B cells via the action of the endosomal protease cathepsin D, and this design feature was critical for T cell activation. Antigens with high BCR epitope valency induce greater BCR-mediated internalization and T cell activation than did low valency antigens, and these high-valency polymeric antigens were superior to protein antigens. We anticipate that these findings can guide the design of more effective vaccines.

Synthetic antigens reveal dynamics of BCR endocytosis during inhibitory signaling

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Courtney, A.H., Bennett, N.R., Zwick, D.B., Hudon, J., Kiessling, L.L.

ACS Chemical Biology. 2014, 9 (1), 202-210.

Efficacious vaccines require antigens that elicit productive immune system activation. Antigens that afford robust antibody production activate both B and T cells. Elucidating the antigen properties that enhance B-T cell communication is difficult with traditional antigens. We therefore used ring-opening metathesis polymerization to access chemically defined, multivalent antigens containing both B and T cell epitopes to explore how antigen structure impacts B cell and T cell activation and communication. The bifunctional antigens were designed so that the backbone substitution level of each antigenic epitope could be quantified using (19)F NMR. The T cell peptide epitope was appended so that it could be liberated in B cells via the action of the endosomal protease cathepsin D, and this design feature was critical for T cell activation. Antigens with high BCR epitope valency induce greater BCR-mediated internalization and T cell activation than did low valency antigens, and these high-valency polymeric antigens were superior to protein antigens. We anticipate that these findings can guide the design of more effective vaccines.

A high affinity RGD-based peptidomemetic supports cell adhesion and activation of aVb3 integrins

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Klim, J.R., Fowler, A. J., Courtney, A.H., Wrighton, P.J., Sheridan, R.T.C., Wong, M.L., Kiessling, L.L.,

ACS Chemical Biology. 2012, 7 (3), 518-525.

Integrins play myriad and vital roles in development and disease. They connect a cell with its surroundings and transmit chemical and mechanical signals across the plasma membrane to the cell's interior. Dissecting their roles in cell behavior is complicated by their overlapping ligand specificity and shared downstream signaling components. In principle, immobilized synthetic peptides can mimic extracellular matrix proteins by supporting integrin-mediated adhesion, but most short peptide sequences lack selectivity for one integrin over others. In contrast, synthetic integrin antagonists can be highly selective. We hypothesized that this selectivity could be exploited if antagonists, when immobilized, could support cellular adhesion and activate signaling by engaging specific cell-surface integrins. To investigate this possibility, we designed a bifunctional (RGD)-based peptidomimetic for surface presentation. Our conjugate combines a high affinity integrin ligand with a biotin moiety; the former engages the α(v)β(3) integrin, and the latter allows for presentation on streptavidin-coated surfaces. Surfaces decorated with this ligand promote both cellular adhesion and integrin activation. Moreover, the selectivity of these surfaces for the α(v)β(3) integrin can be exploited to capture a subset of cells from a mixed population. We anticipate that surfaces displaying highly selective small molecule ligands can reveal the contributions of specific integrin heterodimers to cell adhesion and signaling.

Spatial control of cell fate using synthetic surfaces to potentiate TGF-b signalling

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Li, L., Klim, J.R., Derda, R., Courtney, A.H., Kiessling, L.L.,

Proceedings of the National Academy of Sciences USA. 2011, 108 (29), 11745-11750.

(Highlighted in C&E News)

In organisms, cell-fate decisions result from external cues presented by the extracellular microenvironment or the niche. In principle, synthetic niches can be engineered to give rise to patterned cell signaling, an advance that would transform the fields of tissue engineering and regenerative medicine. Biomaterials that display adhesive motifs are critical steps in this direction, but promoting localized signaling remains a major obstacle. We sought to exert precise spatial control over activation of TGF-β signaling. TGF-β signaling, which plays fundamental roles in development, tissue homeostasis, and cancer, is initiated by receptor oligomerization. We therefore hypothesized that preorganizing the transmembrane receptors would potentiate local TGF-β signaling. To generate surfaces that would nucleate the signaling complex, we employed defined self-assembled monolayers that present peptide ligands to TGF-β receptors. These displays of nondiffusible ligands do not compete with the growth factor but rather sensitize bound cells to subpicomolar concentrations of endogenous TGF-β. Cells adhering to the surfaces undergo TGF-β-mediated growth arrest and the epithelial to mesenchymal transition. Gene expression profiles reveal that the surfaces selectively regulate TGF-β responsive genes. This strategy provides access to tailored surfaces that can deliver signals with spatial control.

Sialylated multivalent antigens engage CD22 in trans and inhibit B cell activation

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Courtney, A.H., Puffer, E.B., Pontrello, J.K., Yang, Z.Q., Kiessling L.L.,

Proceedings of the National Academy of Sciences USA. 2009, 106 (8), 2500-2505.

(Highlighted in C&E News and ACS Chemical Biology)

CD22 is an inhibitory coreceptor on the surface of B cells that attenuates B cell antigen receptor (BCR) signaling and, therefore, B cell activation. Elucidating the molecular mechanisms underlying the inhibitory activity of CD22 is complicated by the ubiquity of CD22 ligands. Although antigens can display CD22 ligands, the receptor is known to bind to sialylated glycoproteins on the cell surface. The propinquity of CD22 and cell-surface glycoprotein ligands has led to the conclusion that the inhibitory properties of the receptor are due to cis interactions. Here, we examine the functional consequences of trans interactions by employing sialylated multivalent antigens that can engage both CD22 and the BCR. Exposure of B cells to sialylated antigens results in the inhibition of key steps in BCR signaling. These results reveal that antigens bearing CD22 ligands are powerful suppressors of B cell activation. The ability of sialylated antigens to inhibit BCR signaling through trans CD22 interactions reveals a previously unrecognized role for the Siglec-family of receptors as modulators of immune signaling.

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