Summary Ligation of the CD28 receptor on T cells provides a critical second signal alongside T cell receptor (TCR) ligation for naive T cell activation. Here we discuss the expression, structure, and biochemistry of CD28 and its ligands. CD28 signals play a key role in many T cell processes including cytoskeletal remodeling, production of cytokines, survival, and differentiation. CD28 ligation leads to unique epigenetic, transcriptional, and post-translational changes in T cells that cannot be recapitulated by TCR ligation alone. We discuss the function of CD28 and its ligands in both effector and regulatory T cells. CD28 is critical for regulatory T cell survival and the maintenance of immune homeostasis. We outline the roles that CD28 and its family members play in human disease and we review the clinical efficacy of drugs that block CD28 ligands. Despite the centrality of CD28 and its family members and ligands to immune function, many aspects of CD28 biology remain unclear. Translation of a basic understanding of CD28 function into immunomodulatory therapeutics has been uneven with both successes and failures. Such real-world results may stem from multiple factors including complex receptor-ligand interactions among CD28 family members, differences between the mouse and human CD28 families, and cell-type specific roles of CD28 family members.

Successful metastasis requires the co-evolution of stromal and cancer cells. We used stable isotope labeling of amino acids in cell culture coupled with quantitative, label-free phosphoproteomics to study the bidirectional signaling in ovarian cancer cells and human-derived, cancer-associated fibroblasts (CAFs) after co-culture. In cancer cells, the interaction with CAFs supported glycogenolysis under normoxic conditions and induced phosphorylation and activation of phosphoglucomutase 1, an enzyme involved in glycogen metabolism. Glycogen was funneled into glycolysis, leading to increased proliferation, invasion, and metastasis of cancer cells co-cultured with human CAFs. Glycogen mobilization in cancer cells was dependent on p38α MAPK activation in CAFs. In vivo, deletion of p38α in CAFs and glycogen phosphorylase inhibition in cancer cells reduced metastasis, suggesting that glycogen is an energy source used by cancer cells to facilitate metastatic tumor growth.

Antibody Mediated Rejection (AMR) is associated with a variety of graft-reactive antibodies following kidney transplant. To characterize these antibodies, we immortalized 107 B cell clones from a patient with AMR. In a previous study, we showed that six clones were reacting to multiple self-antigens as well as to HLA and MICA for two of them, thus displaying a pattern of polyreactivity. We show here that all 6 polyreactive clones also reacted to apoptotic but not viable cells. More generally we observed a nearly perfect overlap between polyreactivity and reactivity to apoptotic cells. Functionally, polyreactive antibodies can activate complement, resulting in the deposition of C3d and C4d at the surface of target cells. Testing the serum of 88 kidney transplant recipients revealed a significantly higher IgG reactivity to apoptotic cells in AMR patients than in patients with stable graft function. Moreover, total IgG purified from AMR patients had increased complement activating properties compared to IgG from non-AMR patients. Overall, our studies show the development of polyreactive antibodies cross-reactive to apoptotic cells during AMR. Further studies are now warranted to determine their contribution to the detection of C4d in graft biopsies as well as their role in the pathophysiology of AMR.

Antibody rejection is often accompanied by non-donor HLA specific antibodies (NDSA) and self-reactive antibodies that develop alongside donor-specific antibodies (DSA). To determine the source of these antibodies, we immortalized 107 B cell clones from a kidney transplant recipient with humoral rejection. Two of these clones reacted to HLA class I or MICA. Both clones were also reactive to self antigens and a lysate of a kidney cell line, hence revealing a pattern of polyreactivity. Monoclonality was verified by the identification of a single rearranged immunoglobulin heavy chain variable region (VH) sequence for each clone. By tracking their unique CDR3 sequence, we found that one such polyreactive clone was highly expanded in the patient blood, representing ~0.2% of circulating B cells. The VH sequence of this clone showed evidence of somatic mutations that were consistent with its memory phenotype and its expansion. Lastly, the reactivity of the expanded polyreactive B cell clone was found in the patient serum at time of rejection. In conclusion, we provide here proof of principle at the clonal level that human antibodies can cross-react to HLA and self. Our findings strongly suggest that polyreactive antibodies contribute to DSA, NDSA as well as autoantibodies, in transplant recipients.

Competing positive and negative signaling feedback pathways play a critical role in tuning the sensitivity of T cell receptor activation by creating an ultrasensitive, bistable switch to selectively enhance responses to foreign ligands while suppressing signals from self peptides. In response to T cell receptor agonist engagement, ERK is activated to positively regulate T cell receptor signaling through phosphorylation of Ser59 Lck. To obtain a wide-scale view of the role of ERK in propagating T cell receptor signaling, a quantitative phosphoproteomic analysis of 322 tyrosine phosphorylation sites by mass spectrometry was performed on the human Jurkat T cell line in the presence of U0126, an inhibitor of ERK activation. Relative to controls, U0126-treated cells showed constitutive decreases in phosphorylation through a T cell receptor stimulation time course on tyrosine residues found on upstream signaling proteins (CD3 chains, Lck, ZAP-70), as well as downstream signaling proteins (VAV1, PLCγ1, Itk, NCK1). Additional constitutive decreases in phosphorylation were found on the majority of identified proteins implicated in the regulation of actin cytoskeleton pathway. Although the majority of identified sites on T cell receptor signaling proteins showed decreases in phosphorylation, Tyr598 of ZAP-70 showed elevated phosphorylation in response to U0126 treatment, suggesting differential regulation of this site via ERK feedback. These findings shed new light on ERK’s role in positive feedback in T cell receptor signaling and reveal novel signaling events that are regulated by this kinase, which may fine tune T cell receptor activation.

Mitogen-activated protein kinase phosphatase 3 (MKP-3) is a negative regulator of extracellular signal–related kinase signaling. Our laboratory recently demonstrated that MKP-3 plays an important role in obesity-related hyperglycemia by promoting hepatic glucose output. This study shows that MKP-3 deficiency attenuates body weight gain induced by a high-fat diet (HFD) and protects mice from developing obesity-related hepatosteatosis. Triglyceride (TG) contents are dramatically decreased in the liver of MKP-3−/− mice fed an HFD compared with wild-type (WT) controls. The absence of MKP-3 also reduces adiposity, possibly by repressing adipocyte differentiation. In addition, MKP-3−/− mice display increased energy expenditure, enhanced peripheral glucose disposal, and improved systemic insulin sensitivity. We performed global phosphoproteomic studies to search for downstream mediators of MKP-3 action in liver lipid metabolism. Our results revealed that MKP-3 deficiency increases the phosphorylation of histone deacetylase (HDAC) 1 on serine 393 by 3.3-fold and HDAC2 on serine 394 by 2.33-fold. Activities of HDAC1 and 2 are increased in the livers of MKP-3−/− mice fed an HFD. Reduction of HDAC1/2 activities is sufficient to restore TG content of MKP-3−/− primary hepatocytes to a level similar to that in WT cells.

In this study, the role of sucrose non-fermenting–related kinase (SNRK) in white adipocyte biology was investigated. SNRK is abundantly expressed in adipose tissue, and the expression level is decreased in obese mice. SNRK expression is repressed by inflammatory signals but increased by insulin sensitizer in cultured adipocytes. In vivo, adipose tissue SNRK expression can be decreased by lipid injection but enhanced by macrophage ablation. Knocking down SNRK in cultured adipocytes activates both JNK and IKKβ pathways as well as promotes lipolysis. Insulin-stimulated Akt phosphorylation and glucose uptake are impaired in SNRK knockdown adipocytes. Phosphoproteomic analysis with SNRK knockdown adipocytes revealed significantly decreased phosphorylation of 49 proteins by 25% or more, which are involved in various aspects of adipocyte function with a clear indication of attenuated mTORC1 signaling. Phosphorylation of 43 proteins is significantly increased by onefold or higher, among which several proteins are known to be involved in inflammatory pathways. The inflammatory responses in SNRK knockdown adipocytes can be partially attributable to defective mTORC1 signaling, since rapamycin treatment activates IKKβ and induces lipolysis in adipocytes. In summary, SNRK may act as a suppressor of adipocyte inflammation and its presence is necessary for maintaining normal adipocyte function.

Vavl, a Rac/Rho guanine nucleotide exchange factor and a critical component of the T cell receptor (TCR) signaling cascade, is rapidly tyrosine phosphorylated in response to T cell activation. Vav1 has established roles in proliferation, cytokine secretion, Ca2+ responses, and actin cytoskeleton regulation, however, its function in the regulation of phosphorylation of TCR components, including the ζ chain, the CD3 δ, ε, γ chains, and the associated kinases Lck, and ZAP-70 is not well established. To obtain a more comprehensive picture of the role of Vav1 in receptor proximal signaling, we performed a wide-scale characterization of Vav1-dependent tyrosine phosphorylation events using quantitative phosphoproteomic analysis of Vav1-deficient T cells across a time course of TCR stimulation. Importantly, this study revealed a new function for Vav1 in the negative feedback regulation of the phosphorylation of immunoreceptor tyrosine-based activation motifs within the ζ chains, CD3 δ, ε, γ chains, as well as activation sites on the critical T cell tyrosine kinases Itk, Lck, and ZAP-70. Our study also uncovered a previously unappreciated role for Vav1 in crosstalk between the CD28 and TCR signaling pathways.