The E3 ubiquitin ligase Cbl-b regulates expansion but not functional activity of self-reactive CD4 T cells

DOCK8-expressing T follicular helper cells newly generated beyond self-organized criticality cause systemic lupus erythematosus

Pathogens including autoantigens all failed to induce systemic lupus erythematosus (SLE). We, instead, studied the integrity of host's immune response that recognized pathogen. By stimulating TCR with an antigen repeatedly to levels that surpass host's steady-state response, self-organized criticality, SLE was induced in mice normally not prone to autoimmunity, wherein T follicular helper (Tfh) cells expressing the guanine nucleotide exchange factor DOCK8 on the cell surface were newly generated. DOCK8⁺Tfh cells passed through TCR re-revision and induced varieties of autoantibody and lupus lesions. They existed in splenic red pulp and peripheral blood of active lupus patients, which subsequently declined after therapy. Autoantibodies and disease were healed by anti-DOCK8 antibody in the mice including SLE-model (NZBxNZW) F1 mice. Thus, DOCK8⁺Tfh cells generated after repeated TCR stimulation by immunogenic form of pathogen, either exogenous or endogenous, in combination with HLA to levels that surpass system's self-organized criticality, cause SLE.

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Autoimmunity seldom takes place under integrated steady-state immune response

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Repeated invasion by pathogen, such as measles virus, is not exceptional but routine in life

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DOCK8+Tfh is generated upon TCR overstimulation by pathogen beyond self-organized criticality

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Newly generated DOCK8+Tfh induces autoantibodies and SLE, i.e., autoimmunity

Immune Regulation by Ubiquitin Tagging as Checkpoint Code

The immune system is equipped with effective machinery to mobilize its activation to defend invading microorganisms, and at the same time, to refrain from attacking its own tissues to maintain immune tolerance. The balance of activation and tolerance is tightly controlled by diverse mechanisms, since breakdown of tolerance could result in disastrous consequences such as the development of autoimmune diseases. One of the mechanisms is by the means of protein ubiquitination, which involves the process of tagging a small peptide ubiquitin to protein substrates. E3 ubiquitin ligases are responsible for catalyzing the final step of ubiquitin-substrate conjugation by specifically recognizing substrates to determine their fates of degradation or functional modification. The ubiquitination process is reversible, which is carried out by deubiquitinating enzymes to release the ubiquitin molecule from the conjugated substrates. Protein ubiquitination and deubiquitination serve as checkpoint codes in many key steps of lymphocyte regulation including the development, activation, differentiation, and tolerance induction. In this chapter, we will discuss a few E3 ligases and deubiquitinating enzymes that are important in controlling immune responses, with emphasis on their roles in T cells.

The role of ubiquitin ligases in the control of organ specific autoimmunity

Diabetes mellitus is characterized by chronic hyperglycemia caused by a deficiency in insulin action, insulin secretion or both. Type 1 diabetes is classified as the destruction of beta cells leading to a deficiency in insulin production. Type1 diabetes accounts for 5-10% of patients with diabetes and most commonly is caused by the autoimmune destruction of the beta cells in the pancreas. The adaptive immune system is composed of antigen specific T and B lymphocytes which play a central role in protecting the human body from infectious pathogens but occasionally autoreactive T and B cells can escape immune tolerance, become activated and induce autoimmune diseases. Naïve T cells require two distinct signals one delivered via the antigen receptor and the second through the costimulatory receptor CD28 that leads to the induction of IL-2 gene transcription. IL-2 is an important T cell growth factor that can influence both immunity and tolerance. Given its pivotal role it is not surprising that the immune system places strict regulation over Il2 gene transcription that is controlled by a number of E3 ubiquitin ligases that modulate TCR and CD28 signaling. This review will examine how different E3 ligases function to control T effector cell differentiation and how studies in gene knockout animal models has been crucial in understanding how these proteins function in vivo to regulate immune tolerance in the peripheral circulation.

Abrogating Cbl-b in effector CD8(+) T cells improves the efficacy of adoptive therapy of leukemia in mice

The clinical use of adoptive immunotherapy with tumor-reactive T cells to treat established cancers is limited in part by the poor in vivo survival and function of the transferred T cells. Although administration of exogenous cytokines such as IL-2 can promote T cell survival, such strategies have many nonspecific activities and are often associated with toxicity. We show here that abrogating expression of Casitas B-lineage lymphoma b (Cbl-b), a negative regulator of lymphocyte activation, in tumor-reactive CD8(+) T cells expanded ex vivo increased the efficacy of adoptive immunotherapy of disseminated leukemia in mice. Mechanistically, Cbl-b abrogation bypassed the requirement for exogenous IL-2 administration for tumor eradication in vivo. In addition, CD8(+) T cells lacking Cbl-b demonstrated a lower threshold for activation, better survival following target recognition and stimulation, and enhanced proliferative responses as a result of both IL-2-dependent and -independent pathways. Importantly, siRNA knockdown of Cbl-b in human CD8(+)CD28- effector T cell clones similarly restored IL-2 production and proliferation following target recognition independent of exogenous IL-2, enhanced IFN-γ production, and increased target avidity. Thus, abrogating Cbl-b expression in effector T cells may improve the efficacy of adoptive therapy of some human malignancies.

Anergic CD4+ T cells form mature immunological synapses with enhanced accumulation of c-Cbl and Cbl-b

CD4(+) T cell recognition of MHC:peptide complexes in the context of a costimulatory signal results in the large-scale redistribution of molecules at the T cell-APC interface to form the immunological synapse. The immunological synapse is the location of sustained TCR signaling and delivery of a subset of effector functions. T cells activated in the absence of costimulation are rendered anergic and are hyporesponsive when presented with Ag in the presence of optimal costimulation. Several previous studies have looked at aspects of immunological synapses formed by anergic T cells, but it remains unclear whether there are differences in the formation or composition of anergic immunological synapses. In this study, we energized primary murine CD4(+) T cells by incubation of costimulation-deficient, transfected fibroblast APCs. Using a combination of TCR, MHC:peptide, and ICAM-1 staining, we found that anergic T cells make mature immunological synapses with characteristic central and peripheral supramolecular activation cluster domains that were indistinguishable from control synapses. There were small increases in total phosphotyrosine at the anergic synapse along with significant decreases in phosphorylated ERK 1/2 accumulation. Most striking, there was specific accumulation of c-Cbl and Cbl-b to the anergic synapses. Cbl-b, previously shown to be essential in anergy induction, was found in both the central and the peripheral supramolecular activation clusters of the anergic synapse. This Cbl-b (and c-Cbl) accumulation at the anergic synapse may play an important role in anergy maintenance, induction, or both.