Over the past 20 years great progress has been made in defining most of the key signalling pathways that functionally regulate immune cells. Recently, it has become clear that scaffold proteins have a crucial role in regulating many of these signalling cascades. By binding two or more components of a signalling pathway, scaffold proteins can help to localize signalling molecules to a specific part of the cell or to enhance the efficacy of a signalling pathway. Scaffold proteins can also affect the thresholds and the dynamics of signalling reactions by coordinating positive and negative feedback signals. In this Review, we focus on recent progress in the understanding of the function of scaffold proteins in immune cells.

Because mutations in RAS and BRAF represent the most common mutations found in human tumors, identification of inhibitors has been a major goal. Surprisingly, new oncogenic BRAF specific inhibitors inhibit cells transformed with mutated BRAF but paradoxically stimulate the growth of cells transformed with RAS. Here, we show that the mechanism for activation is via drug-induced dimer formation between CRAF and kinase suppressor of Ras (KSR)1. To understand the function of KSR1, we generated a KSR1 mutant that cannot bind ATP but stabilizes the closed, active conformation of KSR1. Molecular modeling suggested that the mutant stabilizes the two hydrophobic spines critical for the closed active conformation. We, therefore, could use the mutant to discriminate between the scaffold versus kinase functions of KSR1. The KSR1 mutant bound constitutively to RAF and mitogen-activated protein kinase kinase (MEK) but could not reconstitute activity suggesting that the catalytic activity of KSR1 is required for its function. Analogous mutations in BRAF and CRAF allowed us to test the generality of the model. The mutation induced changes consistent with the active, closed conformation of both kinases and confirmed that BRAF functions distinctly from CRAF in the MAP kinase pathway. Not only does this work suggest that KSR1 may function as a kinase, we anticipate that the mutation that we generated may be broadly applicable to stabilize the closed conformation of other kinases many of which may also form dimers.cancer | protein kinase | signal transduction

Growth hormone (GH) is an underappreciated but important regulator of T cell development that can reverse age-related declines in thymopoiesis in rodents. Here, we report findings of a prospective randomized study examining the effects of GH on the immune system of HIV-1-infected adults. GH treatment was associated with increased thymic mass. In addition, GH treatment enhanced thymic output, as measured by both the frequency of T cell receptor rearrangement excision circles in circulating T cells and the numbers of circulating naive and total CD4 + T cells. These findings provide compelling evidence that GH induces de novo T cell production and may, accordingly, facilitate CD4 + T cell recovery in HIV-1-infected adults. Further, these randomized, prospective data have shown that thymic involution can be pharmacologically reversed in humans, suggesting that immune-based therapies could be used to enhance thymopoiesis in immunodeficient individuals.

PEA-15 is a death effector domain-containing phosphoprotein that binds ERK and restricts it to the cytoplasm. PEA-15 also binds to FADD and thereby blocks apoptosis induced by death receptors. Abnormal expression of PEA-15 is associated with type II diabetes and some cancers; however, its physiological function remains unclear. To determine the function of PEA-15 in vivo, we used C57BL/6 mice in which the PEA-15 coding region was deleted. We thereby found that PEA-15 regulates T-cell proliferation. PEA-15-null mice did not have altered thymic or splenic lymphocyte cellularity or differentiation. However, PEA-15 deficient T cells had increased CD3/CD28-induced nuclear translocation of ERK and increased activation of IL-2 transcription and secretion in comparison to control wild-type littermates. Indeed, activation of the T-cell receptor in wild-type mice caused PEA-15 release of ERK. In contrast, overexpression of PEA-15 in Jurkat T cells blocked nuclear translocation of ERK and IL-2 transcription. Finally, PEA-15-null T cells showed increased IL-2 dependent proliferation on stimulation. No differences in T cell susceptibility to apoptosis were found. Thus, PEA-15 is a novel player in T-cell homeostasis. As such this work may have far reaching implications in understanding how the immune response is controlled.

Polarization of T cells involves reorientation of the microtubule-organizing center (MTOC). Because activated ERK is localized at the immunological synapse, we investigated its role by showing that ERK activation is important for MTOC polarization. Suspecting that ERK phosphorylates a regulator of microtubules, we next focused on stathmin, a known ERK substrate. Our work indicates that during T cell activation, ERK is recruited to the synapse allowing it to phosphorylate stathmin molecules near the immunological synapse. Supporting an important role of stathmin phosphorylation in T cell activation, we showed that T cell activation results in increased microtubule growth rate dependent on the presence of stathmin. The significance of this finding was demonstrated by results showing that CTL from stathmin−/− mice displayed defective MTOC polarization and defective target cell cytolysis. These data implicate stathmin as a regulator of the microtubule network during T cell activation.

Targeting CD40 with agonist antibodies is a promising approach to cancer immunotherapy. CD40 acts as a master regulator of immunity by mobilizing multiple arms of the immune system to initiate highly effective CD8 + T-cell-mediated responses against foreign pathogens and tumors. The clinical development of CD40 agonist antibodies requires careful optimization of the antibody to maximize therapeutic efficacy while minimizing adverse effects. Both epitope specificity and isotype are critical for CD40 agonist antibody mechanism of action and potency. We developed a novel antibody, APX005M, which binds with high affinity to the CD40 ligand-binding site on CD40 and is optimized for selective interaction with Fcγ receptors to enhance agonistic potency while limiting less desirable Fc-effector functions like antibody-dependent cellular cytotoxicity of CD40-expressing immune cells. APX005M is a highly potent inducer of innate and adaptive immune effector responses and represents a promising CD40 agonist antibody for induction of an effective anti-tumor immune response with a favorable safety profile.

BackgroundA significant number of melanoma patients treated with anti-PD-1 alone or in combination with anti-CTLA-4 have transient or no response to treatment. Sotigalimab is a CD40 agonist antibody with unique epitope specificity and Fc receptor binding profile for optimal therapeutic application. Preclinical studies suggest that sotigalimab can be combined with PD-1 blockade to trigger effective anti-tumor immunity. We conducted a multi-center, open label, Phase Ib-parallel arm phase II trial (NCT03123783) to evaluate the combination of sotigalimab with nivolumab in subjects with anti-PD-1/PD-L1 refractory metastatic melanoma.MethodsThe study objective was to evaluate the efficacy and safety of sotigalimab in combination with nivolumab in anti-PD-1/PD-L1 refractory advanced melanoma patients. Subjects received sotigalimab (0.3mg/kg) combined with nivolumab (360mg) every 3 weeks. Thirty-eight subjects with unresectable or metastatic melanoma who had confirmed progressive disease during treatment with anti-PD-1 therapy (documented by 2 consecutive tumor assessments) were enrolled (evaluable for safety) and 33 subjects were evaluable for efficacy.ResultsSix subjects had PR (including one unconfirmed PR) for an ORR of 18%. The mDOR was 18.7 months. Two subjects with PR received treatment for >2 years. Three of the six responding subjects remain off all therapy for ≥26 months, and one patient required stereotactic radiosurgery to a single brain lesion ten months after stopping therapy and has not required additional local or systemic therapy since. Three additional subjects had prolonged SD (12.6, 7.6, 6.2 months). The DCR was 48% and 33% of subjects experienced reduction in target lesions. Efficacy was observed in patients regardless of their tumor PD-L1 expression. The overall safety profile of the combination is consistent with the profiles of individual agents. The majority of AEs observed were of mild to moderate intensity (CTCAE Grade ≤2). The most commonly observed AEs were: pyrexia, chills, nausea, fatigue, pruritus, transaminitis, headache, asthenia, myalgia, rash, vomiting and arthralgia. There were no Grade 4 or 5 AEs related to study drugs. There were no treatment discontinuations due to AEs.ConclusionsThe combination of sotigalimab and nivolumab demonstrated treatment benefit (tumor response or prolonged disease control) in anti-PD-1/PD-L1 refractory melanoma patients with an overall favorable safety and tolerability profile. Notably, a subset of patients remain in response off treatment for ≥26 months. These results warrant further study of this combination in advanced, refractory melanoma.AcknowledgementsWe extend our gratitude to the patients and their families who made this trial possible and the clinical study teams involved in this trial. We thank BMS for providing the nivolumab for this study.Trial RegistrationNCT03123783Ethics ApprovalThis study was approved by the Institutional Review Boards at Yale University (#20170300), University of Nebraska Medical Center (#543-18-CB) and The Hospital Regional de Málaga (#19.03.1341E1-GHM).

The mammalian target of rapamycin (mTOR) kinase is a critical regulator of the differentiation of helper and regulatory CD4+ T cells, as well as memory CD8+ T cells. In this study, we investigated the role of the ERK signaling pathway in regulating mTOR activation in T cells. We showed that activation of ERK following TCR engagement is required for sustained mTOR complex 1 (mTORC1) activation. Absence of kinase suppressor of Ras 1 (KSR1), a scaffold protein of the ERK signaling pathway, or inhibition of ERK resulted in decreased mTORC1 activity following T cell activation. However, KSR1-deficient mice displayed normal regulatory CD4+ T cell development, as well as normal memory CD8+ T cell responses to LCMV and Listeria monocytogenes infection. These data indicate that despite its role in mTORC1 activation, KSR1 is not required in vivo for mTOR-dependent T cell differentiation.