IMMUNOLOGY. Moving CTLA-4 from the trash to recycling

Advances in Immunotherapy and Targeted Therapy of Malignant Melanoma

Malignant melanoma (MM) is a malignant tumor, resulting from mutations in melanocytes of the skin and mucous membranes. Its mortality rate accounts for 90% of all dermatologic tumor mortality. Traditional treatments such as surgery, chemotherapy, and radiotherapy are unable to achieve the expected results due to MM's low sensitivity, high drug resistance, and toxic side effects. As treatment advances, immunotherapy and targeted therapy have made significant breakthroughs in the treatment of MM and have demonstrated promising application prospects. However, the heterogeneity of tumor immune response causes more than half of patients to not benefit from clinical immunotherapy and targeted therapy, which delays the patient's condition and causes them to suffer adverse immune events' side effects. The combination of immunotherapy and targeted therapy can help improve therapeutic effects, delay drug resistance, and mitigate adverse effects. This review provides a comprehensive overview of the current development status and research progress of immune checkpoints, targeted genes, and their inhibitors, with a view to providing a reference for the clinical treatment of MM.

mTOR Deletion Alleviates CD4+ T-Cell Dysfunction in Sepsis through Reducing CTLA4 Accumulation Mediated by Rescuing Autophagy

Sepsis has been the leading cause of death in ICU patients. CD4+ T cells are the mainstay of the body's immune system, and the depletion of CD4+ T cells in sepsis is of great concern. Cytotoxic T lymphocyte-associated protein 4 (CTLA4) is a negative immunomodulator for T cell activation and degradation through the autophagy-lysosome pathway. Mammalian target of rapamycin (mTOR) is the most classical upstream regulator of autophagy. With a mouse model of sepsis through cecal ligation and puncture (CLP), T cell specific-mTOR/tuberous sclerosis complex 1 (TSC1)-knockout mice, and bafilomycin A1, a specific autophagosome-lysosome (A-L) fusion inhibitor, we primarily proved that mTOR could modulate the expression and accumulation of CTLA4 by regulating the onset process of autophagy such as A-L fusion. Given such a regulatory relationship, targeting mTOR could provide new light to improve immune function in sepsis, and the prospect of using rapamycin in the clinic would be worth exploring further.

CTLA-4 expression on CD4+ lymphocytes in patients with sepsis-associated immunosuppression and its relationship to mTOR mediated autophagic-lysosomal disorder

Background

The aim of this study was to clarify the relationship between expression level of CTLA-4 on CD4+ T cells and sepsis-associated immunosuppression (SAI), and to elucidate the possible mechanism of mTOR pathway mediated autophagic-lysosomal disorder in regulating CTLA-4 expression.

Methods

We enrolled 63 sepsis patients admitted to our ICU between January 1 and June 30, 2023. Peripheral blood mononuclear cells were isolated from the patients within 24 hours of recruitment. Expression levels of mTOR, P62, LC3II, and CTLA-4 on circulating CD4+ T lymphocytes were quantitated using flow cytometry. The association of these markers and relationship between CTLA-4 expression and the incidence of SAI and 28-day mortality were comprehensively analyzed.

Results

Compared with non-immunosuppressed patients with sepsis, patients with SAI had a higher 28-day mortality rate (37.5% vs 13.0%, P=0.039) and higher CTLA-4 mean fluorescence intensity (MFI) on CD4+ T cells (328.7 versus 78.7, P<0.0001). CTLA-4 MFI on CD4+ cells was independently associated with the occurrence of SAI (95% confidence interval: 1.00-1.14, P=0.044). In patients with sepsis and SAI, non-survivors had higher CTLA-4 expression than survivors (sepsis: 427.5 versus 130.6, P=0.002; and SAI: 506.7 versus 225.2, P<0.0001). The sensitivity and specificity of CTLA-4 MFI at predicting 28-day mortality in patients with SAI was 100% and 80% respectively with the cutoff value of 328.7 and the area under the curve of 0.949. The MFI of mTOR, P62, and LC3II on CD4+ T cells were statistically higher in patients with SAI than in non-immunosuppressed patients (267.2 versus 115.9, P<0.0001; 314.8 versus 173.7, P<0.0001; and 184.7 versus 1123.5, P=0.012, respectively); P62 and LC3II were markedly higher in non-survivors than in survivors of sepsis (302.9 versus 208.9, P=0.039; and 244.3 versus 122.8, P<0.0001 respectively). The expression of CTLA-4 statistically correlated with that of LC3II in patients with sepsis, patients with SAI, and patients with SAI who did not survive (correlation coefficient: 0.69, 0.68, and 0.73, respectively, P<0.0001).

Conclusions

CTLA-4 overexpression on CD4+ T cells was markedly associated with the incidence of SAI and had great relevance to 28-day mortality. mTOR pathway mediated autophagic-lysosomal disorder showed significant association with CTLA-4 expression.

Functional Relevance of CTLA4 Variants: an Upgraded Approach to Assess CTLA4-Dependent Transendocytosis by Flow Cytometry

Variants of uncertain significance (VUS) in CTLA4 are frequently identified in patients with antibody deficiency or immune dysregulation syndromes including, but not limited to, patients with multi-organ autoimmunity and autoinflammation. However, to ascertain the diagnosis of CTLA4 insufficiency, the functional relevance of each variant needs to be determined. Currently, various assays have been proposed to assess the functionality of CTLA4 VUS, including the analysis of transendocytosis, the biological function of CTLA4 to capture CD80 molecules from antigen presenting cells. Challenges of this assay include weak fluorescence intensity of the internalized ligand, poor reproducibility, and poor performance upon analyzing thawed cells. In addition, the distinction of pathogenic from non-pathogenic variants and from wild-type CTLA4, and the classification of the different VUS according to its level of CTLA4 dysfunction, would be desirable. We developed a novel CD80-expressing cell line for the evaluation of CD80-transendocytosis and compared it to the published transendocytosis assay. Our approach showed lower inter-assay variability and better robustness regardless the type of starting material (fresh or thawed peripheral mononuclear cells). In addition, receiver operating characteristic analysis showed 100% specificity, avoiding false positive results and allowing for a clear distinction between pathogenic and non-pathogenic variants in CTLA4-variant carriers. With our transendocytosis assay, we assessed the pathogenicity of 24 distinct CTLA4 variants from patients submitted to our diagnostic unit. Significantly impaired transendocytosis was demonstrated for 17 CTLA4 variants, whereas seven variants tested normal. In conclusion, our upgraded transendocytosis assay allows a reliable assessment of newly identified variants in CTLA4.

CTLA-4 tail fusion enhances CAR-T antitumor immunity

Chimeric antigen receptor (CAR)-T cells are powerful therapeutics; however, their efficacy is often hindered by critical hurdles. Here utilizing the endocytic feature of the cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) cytoplasmic tail, we reprogram CAR function and substantially enhance CAR-T efficacy in vivo. CAR-T cells with monomeric, duplex or triplex CTLA-4 cytoplasmic tails (CCTs) fused to the C terminus of CAR exhibit a progressive increase in cytotoxicity under repeated stimulation, accompanied by reduced activation and production of proinflammatory cytokines. Further characterization reveals that CARs with increasing CCT fusion show a progressively lower surface expression, regulated by their constant endocytosis, recycling and degradation under steady state. The molecular dynamics of reengineered CAR with CCT fusion results in reduced CAR-mediated trogocytosis, loss of tumor antigen and improved CAR-T survival. CARs with either monomeric (CAR-1CCT) or duplex CCTs (CAR-2CCT) have superior antitumor efficacy in a relapsed leukemia model. Single-cell RNA sequencing and flow cytometry analysis reveal that CAR-2CCT cells retain a stronger central memory phenotype and exhibit increased persistence. These findings illuminate a unique strategy for engineering therapeutic T cells and improving CAR-T function through synthetic CCT fusion, which is orthogonal to other cell engineering techniques.

CTLA-4 tail fusion enhances CAR-T anti-tumor immunity

Chimeric antigen receptor (CAR) T cells are powerful therapeutics; however, their efficacy is often hindered by critical hurdles. Here, utilizing the endocytic feature of the cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) cytoplasmic tail (CT), we reprogram CAR function and substantially enhance CAR-T efficacy in vivo . CAR-T cells with monomeric, duplex, or triplex CTLA-4 CTs (CCTs) fused to the C-terminus of CAR exhibit a progressive increase in cytotoxicity under repeated stimulation, accompanied by reduced activation and production of pro-inflammatory cytokines. Further characterization reveals that CARs with increasing CCT fusion show a progressively lower surface expression, regulated by their constant endocytosis, recycling and degradation under steady state. The molecular dynamics of reengineered CAR with CCT fusion results in reduced CAR-mediated trogocytosis, loss of tumor antigen, and improved CAR-T survival. CARs with either monomeric (CAR-1CCT) or duplex CCTs (CAR-2CCT) have superior anti-tumor efficacy in a relapsed leukemia model. Single-cell RNA sequencing and flow cytometry analysis reveal that CAR-2CCT cells retain a stronger central memory phenotype and exhibit increased persistence. These findings illuminate a unique strategy for engineering therapeutic T cells and improving CAR-T function through synthetic CCT fusion, which is orthogonal to other cell engineering techniques.

Differences in CD80 and CD86 transendocytosis reveal CD86 as a key target for CTLA-4 immune regulation

CD28 and CTLA-4 (CD152) play essential roles in regulating T cell immunity, balancing the activation and inhibition of T cell responses, respectively. Although both receptors share the same ligands, CD80 and CD86, the specific requirement for two distinct ligands remains obscure. In the present study, we demonstrate that, although CTLA-4 targets both CD80 and CD86 for destruction via transendocytosis, this process results in separate fates for CTLA-4 itself. In the presence of CD80, CTLA-4 remained ligand bound, and was ubiquitylated and trafficked via late endosomes and lysosomes. In contrast, in the presence of CD86, CTLA-4 detached in a pH-dependent manner and recycled back to the cell surface to permit further transendocytosis. Furthermore, we identified clinically relevant mutations that cause autoimmune disease, which selectively disrupted CD86 transendocytosis, by affecting either CTLA-4 recycling or CD86 binding. These observations provide a rationale for two distinct ligands and show that defects in CTLA-4-mediated transendocytosis of CD86 are associated with autoimmunity.

LRBA is essential for urinary concentration and body water homeostasis

Protein kinase A (PKA) directly phosphorylates aquaporin-2 (AQP2) water channels in renal collecting ducts to reabsorb water from urine for the maintenance of systemic water homeostasis. More than 50 functionally distinct PKA-anchoring proteins (AKAPs) respectively create compartmentalized PKA signaling to determine the substrate specificity of PKA. Identification of an AKAP responsible for AQP2 phosphorylation is an essential step toward elucidating the molecular mechanisms of urinary concentration. PKA activation by several compounds is a novel screening strategy to uncover PKA substrates whose phosphorylation levels were nearly perfectly correlated with that of AQP2. The leading candidate in this assay proved to be an AKAP termed lipopolysaccharide-responsive and beige-like anchor protein (LRBA). We found that LRBA colocalized with AQP2 in vivo, and Lrba knockout mice displayed a polyuric phenotype with severely impaired AQP2 phosphorylation. Most of the PKA substrates other than AQP2 were adequately phosphorylated by PKA in the absence of LRBA, demonstrating that LRBA-anchored PKA preferentially phosphorylated AQP2 in renal collecting ducts. Furthermore, the LRBA-PKA interaction, rather than other AKAP-PKA interactions, was robustly dissociated by PKA activation. AKAP-PKA interaction inhibitors have attracted attention for their ability to directly phosphorylate AQP2. Therefore, the LRBA-PKA interaction is a promising drug target for the development of anti-aquaretics.

Continuous Modeling of T CD4 Lymphocyte Activation and Function

T CD4+ cells are central to the adaptive immune response against pathogens. Their activation is induced by the engagement of the T-cell receptor by antigens, and of co-stimulatory receptors by molecules also expressed on antigen presenting cells. Then, a complex network of intracellular events reinforce, diversify and regulate the initial signals, including dynamic metabolic processes that strongly influence both the activation state and the differentiation to effector cell phenotypes. The regulation of cell metabolism is controlled by the nutrient sensor adenosine monophosphate-activated protein kinase (AMPK), which drives the balance between oxidative phosphorylation (OXPHOS) and glycolysis. Herein, we put forward a 51-node continuous mathematical model that describes the temporal evolution of the early events of activation, integrating a circuit of metabolic regulation into the main routes of signaling. The model simulates the induction of anergy due to defective co-stimulation, the CTLA-4 checkpoint blockade, and the differentiation to effector phenotypes induced by external cytokines. It also describes the adjustment of the OXPHOS-glycolysis equilibrium by the action of AMPK as the effector function of the T cell develops. The development of a transient phase of increased OXPHOS before induction of a sustained glycolytic phase during differentiation to the Th1, Th2 and Th17 phenotypes is shown. In contrast, during Treg differentiation, glycolysis is subsequently reduced as cell metabolism is predominantly polarized towards OXPHOS. These observations are in agreement with experimental data suggesting that OXPHOS produces an ATP reservoir before glycolysis boosts the production of metabolites needed for protein synthesis, cell function, and growth.

Association between CTLA-4 gene polymorphism and risk of rheumatoid arthritis: a meta-analysis

Cytotoxic T lymphocyte-associated protein 4 (CTLA-4) gene polymorphisms may be involved in the risk of Rheumatoid arthritis (RA). However, evidence for the association remains controversial. Therefore, we performed a meta-analysis to confirm the relationship between CTLA-4 gene polymorphisms and RA. The pooled odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to assess the strength of association. Stratified analysis was conducted by ethnicity. In total, 66 case-control studies including 21681 cases and 23457 controls were obtained. For rs3087243 polymorphism, significant association was detected in Asians (A vs. G: OR=0.77, 95%CI=0.65-0.90, P=0.001; AA vs. GG: OR=0.67, 95%CI=0.48-0.94, P=0.02) and Caucasians (A vs. G: OR=0.89, 95%CI=0.86-0.93, P<0.00001; AA vs. GG: OR=0.81, 95%CI=0.75-0.88, P<0.00001). For rs231775 polymorphism, significant association was observed in the overall (G vs. A: OR =1.16, 95%CI=1.08-1.25, P<0.0001; GG vs. AA: OR=1.29, 95%CI=1.12-1.50, P=0.0006), and in Asians (G vs. A: OR=1.27, 95%CI=1.10-1.47, P=0.001; GG vs. AA: OR=1.58, 95%CI=1.24-2.01, P=0.0002), but not in Caucasians. However, there was no association between rs5742909 polymorphism and RA. This meta-analysis confirmed that rs3087243 and rs231775 polymorphism were associated with the risk of RA in both overall population and ethnic-specific analysis, but there was no association between rs5742909 polymorphism and RA risk.

Immune checkpoint deficiencies and autoimmune lymphoproliferative syndromes

Autoimmune lymphoproliferative syndrome (ALPS) is an inherited non-malignant and non-infectious lymphoproliferative syndrome caused by mutations in genes affecting the extrinsic apoptotic pathway (FAS, FASL, CASP10). The resulting FAS-mediated apoptosis defect accounts for the expansion and accumulation of autoreactive (double-negative) T cells leading to cytopenias, splenomegaly, lymphadenopathy, autoimmune disorders, and risk of lymphoma. However, there are other monogenetic disorders known as ALPS-like syndromes that can be clinically similar to ALPS but are genetically and biologically different, such as observed in patients with immune checkpoint deficiencies, particularly cytotoxic T-lymphocyte antigen 4 (CTLA-4) insufficiency and lipopolysaccharide-responsive beige-like anchor protein LRBA deficiency. CTLA-4 insufficiency is caused by heterozygous mutations in CTLA-4, an essential negative immune regulator that is constitutively expressed on regulatory T (Treg) cells. Mutations in CTLA-4 affect CTLA-4 binding to CD80-CD86 costimulatory molecules, CTLA-4 homodimerization, or CTLA-4 intracellular vesicle trafficking upon cell activation. Abnormal CTLA-4 trafficking is also observed in patients with LRBA deficiency, a syndrome caused by biallelic mutations in LRBA that abolishes the LRBA protein expression. Both immune checkpoint deficiencies are biologically characterized by low levels of CTLA-4 protein on the cell surface of Tregs, accounting for the autoimmune manifestations observed in CTLA4-insufficient and LRBA-deficient patients. In addition, both immune checkpoint deficiencies present with an overlapping but heterogeneous clinical picture despite the difference in inheritance and penetrance. In this review, we describe the most prominent clinical features of ALPS, CTLA-4 insufficiency and LRBA deficiency, emphasizing their corresponding biological mechanisms. We also provide some clinical and laboratory approaches to diagnose these three rare immune disorders, together with therapeutic strategies that have worked best at improving prognosis and quality life of patients.

Case Report: Refractory Autoimmune Gastritis Responsive to Abatacept in LRBA Deficiency

Primary immunodeficiency (PID) with immune dysregulation may present with early onset gastrointestinal autoimmune disorders. When gastrointestinal autoimmunity is associated with multiple extraintestinal immune system dysfunction the diagnosis of PID is straightforward. However, with the advent of next generation sequencing technologies, genetic defects in PID genes have been increasingly recognized even when a single or no extraintestinal signs of immune dysregulation are present. A genetic diagnosis is especially important considering the expanding armamentarium of therapies designed to inhibit specific molecular pathways. We describe a boy with early-onset severe, refractory autoimmune gastritis and biallelic mutations in the LRBA gene causing a premature STOP-codon who was successfully treated with CTLA4-Ig, abatacept, with long term clinical and endoscopic remission. The case underscores the importance to consider a monogenetic defect in early onset autoimmune disorders, since the availability of targeted treatments may significantly improve patient prognosis.

Immune checkpoint inhibitor (ICI) combination therapy compared to monotherapy in advanced solid cancer: A systematic review

Aim: To evaluate the efficacy and safety of immune checkpoint inhibitor (ICI) two-drug combination therapy in patients with advanced malignancy. Methods: We searched PubMed, PMC, EMBASE, EBSCO, Cochrane Central Register of Controlled Trials (CENTRAL), American Society of Clinical Oncology (ASCO and the European Society of Medical Oncology (ESMO) to identify primary research reporting the survival outcomes and safety of ICI combination therapy in patients with advanced malignancy. We performed a meta-analysis that evaluated the risk ratio (RR) and its 95% confidence interval (CI) for objective response rates (ORR) and disease control rates (DCR), hazard ratio (HR) and 95% CI for progression-free survival (PFS) and overall survival time (OS), and RR and 95% CI for adverse events (AEs). Results: The final 10 studies (15 cohorts) and 2410 patients were included in the meta-analysis. The ICI combination therapy resulted in improved ORR (RR 1.82, 95% CI 1.31-2.54, p = 0.0004), DCR (RR 1.41, 95% CI 1.29-1.55, p < 0.0001), PFS (HR 0.83, 95% CI 0.74-0.94, p = 0.003) and OS (HR 0.90, 95% CI 0.82-0.98, p = 0.02) in patients with advanced malignant tumors. The incidence of some high grade (≥3) AEs increased, such as fatigue, nausea, diarrhea, colitis, rash, pruritus, elevated transaminase and lipase. Conclusion: Our study showed that ICI combination therapy can improve ORR, DCR, PFS and OS in patients with advanced malignancy. Compared with ICI monotherapy, ICI combination therapy was more likely to induce severe AEs.

Intrahepatic TH17/TReg Cells in Homeostasis and Disease-It's All About the Balance

Both acute and chronic hepatic inflammation likely result from an imbalance in the T_H1/T_H2 cell response and can lead to liver fibrosis and end-stage liver disease. More recently, a novel CD4+ T helper cell subset was described, characterized by the production of IL-17 and IL-22. These T_H17 cells 50were predominantly implicated in host defense against infections and in autoimmune diseases. Interestingly, studies over the last 10 years revealed that the development of T_H17 cells favors pro-inflammatory responses in almost all tissues and there is a reciprocal relationship between T_H17 and T_Reg cells. The balance between T_H17and T_Reg cells is critical for immune reactions, especially in injured liver tissue and the return to immune homeostasis. The pathogenic contribution of T_H17 and T_Reg cells in autoimmunity, acute infection, and chronic liver injury is diverse and varies among disease etiologies. Understanding the mechanisms underlying T_H17 cell development, recruitment, and maintenance, along with the suppression of T_Reg cells, will inform the development of new therapeutic strategies in liver diseases. Active manipulation of the balance between pathogenic and regulatory processes in the liver may assist in the restoration of homeostasis, especially in hepatic inflammation.

What did we learn from CTLA-4 insufficiency on the human immune system?

Cytotoxic-T-lymphocyte-antigen-4 (CTLA-4) is a negative immune regulator constitutively expressed on regulatory T (Treg) cells and upregulated on activated T cells. CTLA-4 inhibits T cell activation by various suppressive functions including competition with CD28, regulation of the inhibitory function of Treg cells, such as transendocytosis, and the control of adhesion and motility. Intrinsic CTLA-4 signaling has been controversially discussed, but so far no distinct signaling pathway has been identified. The CTLA-4-mediated Treg suppression plays an important role in the maintenance of peripheral tolerance and the prevention of autoimmune diseases. Human CTLA-4 insufficiency is caused by heterozygous germline mutations in CTLA4 and characterized by a complex immune dysregulation syndrome. Clinical studies on CTLA4 mutation carriers showed a reduced penetrance and variable expressivity, suggesting modifying factor(s). One hundred and forty-eight CTLA4 mutation carriers have been reported; patients showed hypogammaglobulinemia, recurrent infectious diseases, various autoimmune diseases, and lymphocytic infiltration into multiple organs. The CTLA-4 expression level in Treg cells was reduced, while the frequency of Treg cells was increased in CTLA-4-insufficient patients. The transendocytosis assay is a specific functional test for the assessment of newly identified CTLA4 gene variants. Immunoglobulin substitution, corticosteroids, immunosuppressive therapy, and targeted therapy such as with CTLA-4 fusion proteins and mechanistic target of rapamycin (mTOR) inhibitors were applied; patients with life-threatening, treatment-resistant symptoms underwent hematopoietic stem cell transplantation. The fact that in humans CTLA-4 insufficiency causes severe disease taught us that the amount of CTLA-4 molecules present in/on T cells matters for immune homeostasis. However, whether the pathology-causing activated T lymphocytes in CTLA-4-insufficient patients are antigen-specific is an unsolved question. CTLA-4, in addition, has a role in autoimmune diseases and cancer. Anti-CTLA-4 drugs are employed as checkpoint inhibitors to target various forms of cancer. Thus, clinical research on human CTLA-4 insufficiency might provide us a deeper understanding of the mechanism(s) of the CTLA-4 molecule and immune dysregulation disorders.

Acquired Pure Red Cell Aplasia and Acquired Amegakaryocytic Thrombocytopenia Associated With Clonal Expansion of T-Cell Large Granular Lymphocytes in a Patient With Lipopolysaccharide-responsive Beige-like Anchor (LRBA) Protein Deficiency

Acquired pure red cell aplasia and acquired amegakaryocytic thrombocytopenic purpura are rare in children. Similarly, clonal expansion of T-cell large granular lymphocytes is infrequently seen in pediatrics. Lipopolysaccharide-responsive beige-like anchor (LRBA) protein deficiency is a recently described immunodeficiency syndrome that has been associated with inflammatory bowel disease and autoimmune phenomena such as Evans syndrome. Here, we describe a patient with LRBA deficiency who developed acquired pure red cell aplasia and acquired amegakaryocytic thrombocytopenic purpura associated with expansion of clonal T-cell large granular lymphocytes. This has not been described in the literature previously and adds to the knowledge on the spectrum of manifestations of LRBA deficiency.

Two male siblings with a novel LRBA mutation presenting with different findings of IPEX syndrome

Introduction

LPS-responsive beige-like anchor (LRBA) protein deficiency is a disease of immune dysregulation with autoimmunity affecting various systems.

Case Presentation

Two male siblings with a novel LRBA mutation had different primary findings at admission: the younger sibling had chronic early-onset diarrhoea and the elder one had autoimmune haemolytic anaemia. During long-term follow-up for IPEX phenotype, both developed hypogammaglobulinaemia, enteropathy and lung involvement. The patients partially responded to immunosuppressive therapies. A homozygous c.2496C>A, p.Cys832Ter (p.C832*) mutation in the LRBA gene causing a premature stop codon was detected. After molecular diagnosis, abatacept, as a target-specific molecule, was used with promising results.

Conclusion

LRBA deficiency is a recently defined defect, with variable presentations in different patients; a single, definitive treatment option is thus not yet available.

Characterization of CTLA4 Trafficking and Implications for Its Function

CTLA4 is an essential negative regulator of T-cell immune responses and a key checkpoint regulating autoimmunity and antitumor responses. Genetic mutations resulting in quantitative defects in the CTLA4 pathway are also associated with the development of immune dysregulation syndromes in humans. It has been proposed that CTLA4 functions to remove its ligands CD80 and CD86 from opposing cells by a process known as transendocytosis. A quantitative characterization of CTLA4 synthesis, endocytosis, degradation, and recycling and how these affect its function is currently lacking. In a combined in vitro and in silico study, we developed a mathematical model and identified these trafficking parameters. Our model predicts optimal ligand removal in an intermediate affinity range. The intracellular CTLA4 pool as well as fast internalization, recovery of free CTLA4 from internalized complexes, and recycling is critical for sustained functionality. CD80-CTLA4 interactions are predicted to dominate over CD86-CTLA4. Implications of these findings in the context of control of antigen-presenting cells by regulatory T cells and of pathologic genetic deficiencies are discussed. The presented mathematical model can be reused in the community beyond these questions to better understand other trafficking receptors and study the impact of CTLA4 targeting drugs.

Molecular Classification of Primary Immunodeficiencies of T Lymphocytes

Proper regulation of the immune system is required for protection against pathogens and preventing autoimmune disorders. Inborn errors of the immune system due to inherited or de novo germline mutations can lead to the loss of protective immunity, aberrant immune homeostasis, and the development of autoimmune disease, or combinations of these. Forward genetic screens involving clinical material from patients with primary immunodeficiencies (PIDs) can vary in severity from life-threatening disease affecting multiple cell types and organs to relatively mild disease with susceptibility to a limited range of pathogens or mild autoimmune conditions. As central mediators of innate and adaptive immune responses, T cells are critical orchestrators and effectors of the immune response. As such, several PIDs result from loss of or altered T cell function. PID-associated functional defects range from complete absence of T cell development to uncontrolled effector cell activation. Furthermore, the gene products of known PID causal genes are involved in diverse molecular pathways ranging from T cell receptor signaling to regulators of protein glycosylation. Identification of the molecular and biochemical cause of PIDs can not only guide the course of treatment for patients, but also inform our understanding of the basic biology behind T cell function. In this chapter, we review PIDs with known genetic causes that intrinsically affect T cell function with particular focus on perturbations of biochemical pathways.

Study of an extended family with CTLA-4 deficiency suggests a CD28/CTLA-4 independent mechanism responsible for differences in disease manifestations and severity

The CTLA-4 checkpoint regulates the activation of T cells. Individuals with heterozygous mutations in CTLA-4 have a complex phenotype typically characterized by antibody deficiency alongside variable autoimmunity. Despite severe disease in some individuals, others remain largely unaffected with reasons for this variation unknown. We studied a large family carrying a single point mutation in CTLA-4 leading to an amino acid change R75W and compared both unaffected with affected individuals. We measured a variety of features pertaining to T cell and CTLA-4 biology and observed that at the cellular level there was complete penetrance of CTLA-4 mutations. Accordingly, unaffected individuals were indistinguishable from those with disease in terms of level of CTLA-4 expression, percentage of Treg, upregulation of CTLA-4 upon stimulation and proliferation of CD4 T cells. We conclude that the wide variation in disease phenotype is influenced by immune variation outside of CTLA-4 biology.

Adjuvanticity of a CTLA-4 3' UTR complementary oligonucleotide for emulsion formulated recombinant subunit and inactivated vaccines

Cytotoxic T-lymphocyte antigen 4 (CTLA-4) is recognized as a critical inhibitory regulator of T-cell proliferation and activation, opposing the action of CD28-mediated co-stimulation. Interfering or blocking CTLA-4 can result in continuous T-cell activation required for the full immune response to pathogenic microbes and vaccines. To test if nucleic acid-based CTLA-4 inhibitors could be developed into a novel adjuvant, we designed two oligonucleotides, CMD-1 and CMD-2, with the sequences complementary to the conserve regions identical between human and mouse CTLA-4 mRNA 3' untranslated region (3' UTR), and tested their in vitro effects on CTLA-4 production and their adjuvanticity for vaccines in mice. We found that CMD-1 inhibited the antigen-induced CTLA-4 up-regulation on the CD4⁺ T cells by interfering its mRNA expression, maintained higher levels of CD80 and CD86 on the CD11c⁺ cells and promoted the recalled proliferation of the CD4⁺ T cells and CD19⁺ B cells, and that the CMD-1 enhanced the antibody response against recombinant PCV2b capsid protein or inactivated foot-and-mouth disease virus in both ICR and BALB/c mice. These data suggest that the CMD-1 could be used as a novel vaccine adjuvant capable of inhibiting inhibitory signals rather than inducing stimulatory signals of immune cells.

Identifying functional defects in patients with immune dysregulation due to LRBA and CTLA-4 mutations

Heterozygous CTLA-4 deficiency has been reported as a monogenic cause of common variable immune deficiency with features of immune dysregulation. Direct mutation in CTLA-4 leads to defective regulatory T-cell (Treg) function associated with impaired ability to control levels of the CTLA-4 ligands, CD80 and CD86. However, additional mutations affecting the CTLA-4 pathway, such as those recently reported for LRBA, indirectly affect CTLA-4 expression, resulting in clinically similar disorders. Robust phenotyping approaches sensitive to defects in the CTLA-4 pathway are therefore required to inform understanding of such immune dysregulation syndromes. Here, we describe assays capable of distinguishing a variety of defects in the CTLA-4 pathway. Assessing total CTLA-4 expression levels was found to be optimal when restricting analysis to the CD45RA^-Foxp3⁺ fraction. CTLA-4 induction following stimulation, and the use of lysosomal-blocking compounds, distinguished CTLA-4 from LRBA mutations. Short-term T-cell stimulation improved the capacity for discriminating the Foxp3⁺ Treg compartment, clearly revealing Treg expansions in these disorders. Finally, we developed a functionally orientated assay to measure ligand uptake by CTLA-4, which is sensitive to ligand-binding or -trafficking mutations, that would otherwise be difficult to detect and that is appropriate for testing novel mutations in CTLA-4 pathway genes. These approaches are likely to be of value in interpreting the functional significance of mutations in the CTLA-4 pathway identified by gene-sequencing approaches.

Immune based therapy for melanoma

A few years ago therapeutic options in advanced melanoma were very limited and the prognosis was somber. Although recent progresses are far from providing a cure for advanced melanoma, yet these have kindled new hopes and searching for a cure does not seem unreasonable. Seven new medicines have been authorized in various regions of the world in the recent past in the therapy of advanced melanoma, over half of them acting by mechanisms involving the immune system of the host. The anti-CTLA-4 (cytotoxic T lymphocyte associated protein-4) ipilimumab has been followed by anti-PD1 (programmed death1) inhibitors, more effective and safer. Very recently, the first oncolytic immunotherapy, talimogene laherparepvec (T-VEC) has been authorized for placing on the market and a variety of combinations of the new therapies are currently being evaluated or considered. Besides, a plethora of other molecules and approaches, especially monoclonal antibodies, are in the preliminary phases of clinical investigation and are likely to bring new benefits for the treatment of this potentially fatal form of cancer.