Rag GTPase critically contributes to humoral immunity independent of canonical mTORC1 signaling

The humoral immune response requires that B cells undergo a rapid metabolic shift and high demand of nutrients, which are vital to sustain the formation of germinal center. Rag GTPase senses amino acid availability to activate mechanistic target of rapamycin complex 1(mTORC1) pathway and modulate the function of transcription factor EB (TFEB), a member of the microphthalmia (MiT/TFE) family of HLH-leucine zipper transcription factors. However, little is known about how Rag GTPase coordinates amino acid sensing, mTORC1 activation and TFEB activity in humoral immune response. Here, we show that B cell intrinsic Rag GTPase is critical to the development and activation of B cells. Disruption of Rag GTPase complex, but not mTORC1 complex, abrogates germinal center formation, antibody production as well as plasma cell generation upon respiratory influenza infection. Mechanistically, the Rag GTPase complex senses specific amino acids to suppress TFEB activity, independent of canonical mTORC1 activation. Collectively, our data support the idea that Rag GTPase critically contributes to humoral immunity partly through suppressing TFEB and it is largely not necessary for canonical mTORC1 signaling.

Supported by Mayo Foundation for Medical Education and Research

Over-Generalizing About GC (Hypoxia): Pitfalls of Limiting Breadth of Experimental Systems and Analyses in Framing Informatics Conclusions

Accumulating evidence suggests that many immune responses are influenced by local nutrient concentrations in addition to the programming of intermediary metabolism within immune cells. Humoral immunity and germinal centers (GC) are settings in which these factors are under active investigation. Hypoxia is an example of how a particular nutrient is distributed in lymphoid follicles during an antibody response, and how oxygen sensors may impact the qualities of antibody output after immunization. Using exclusively a bio-informatic analysis of mRNA levels in GC and other B cells, recent work challenged the concept that there is any hypoxia or that it has any influence. To explore this proposition, we performed new analyses of published genomics data, explored potential sources of disparity, and elucidated aspects of the apparently conflicting conclusions. Specifically, replicability and variance among data sets derived from different naïve as well as GC B cells were considered. The results highlight broader issues that merit consideration, especially at a time of heightened focus on scientific reports in the realm of immunity and antibody responses. Based on these analyses, a standard is proposed under which the relationship of new data sets should be compared to prior “fingerprints” of cell types and reported transparently to referees and readers. In light of independent evidence of diversity within and among GC elicited by protein immunization, avoidance of overly broad conclusions about germinal centers in general when experimental systems are subject to substantial constraints imposed by technical features also is warranted.

Selective inhibition of mTORC1 in tumor vessels increases antitumor immunity

A tumor blood vessel is a key regulator of tissue perfusion, immune cell trafficking, cancer metastasis, and therapeutic responsiveness. mTORC1 is a signaling node downstream of multiple angiogenic factors in the endothelium. However, mTORC1 inhibitors have limited efficacy in most solid tumors, in part due to inhibition of immune function at high doses used in oncology patients and compensatory PI3K signaling triggered by mTORC1 inhibition in tumor cells. Here we show that low-dose RAD001/everolimus, an mTORC1 inhibitor, selectively targets mTORC1 signaling in endothelial cells (ECs) without affecting tumor cells or immune cells, resulting in tumor vessel normalization and increased antitumor immunity. Notably, this phenotype was recapitulated upon targeted inducible gene ablation of the mTORC1 component Raptor in tumor ECs (RaptorECKO). Tumors grown in RaptorECKO mice displayed a robust increase in tumor-infiltrating lymphocytes due to GM-CSF-mediated activation of CD103+ dendritic cells and displayed decreased tumor growth and metastasis. GM-CSF neutralization restored tumor growth and metastasis, as did T cell depletion. Importantly, analyses of human tumor data sets support our animal studies. Collectively, these findings demonstrate that endothelial mTORC1 is an actionable target for tumor vessel normalization, which could be leveraged to enhance antitumor immune therapies.

Hypoxia-Inducible Factors (HIF) in CD4+ T cells promote metabolism, switch cytokine secretion, and T cell help in humoral immunity

T cell help in humoral immunity includes interactions of B cells with activated extrafollicular CD4+ and follicular T helper (Tfh) cells. Each can promote antibody responses but Tfh cells play critical roles during germinal center (GC) reactions. After re-stimulation of their antigen receptor (TCR) by B cells, helper T cells act on B cells via CD40 ligand and secreted cytokines that guide immunoglobulin class switching. Recent work showed that hypoxia is a normal feature of most GC, raising questions about molecular mechanisms governing the relationship between hypoxia and T cell help to antibody response. Hypoxia-inducible factors (HIF) are prominent among mechanisms that mediate cellular responses to limited oxygen but also are induced by lymphocyte activation. We now show that loss of HIF-1a and HIF-2a in CD4+ T cells compromised essential functions in help during antibody responses. HIF-1a depletion from CD4 T cells reduced frequencies of antigen-specific GC B cells, Tfh cells, and overall antigen-specific Ab. Compound deficiency of HIF-1a and HIF-2a intensified humoral defects after hapten-carrier immunization. Further, HIF promoted CD40L expression while restraining the FoxP3-positive CD4+ cells in the CCR5+ follicular regulatory (Tfr) population. Glycolysis increases T helper cytokine expression, and HIF was essential for stimulation of glycolysis in T helper cells via TCR or cytokine stimulation, as well as their production of cytokines that direct antibody class switching. Indeed, interferon-g elaboration by HIF-deficient in vivo-generated Tfh cells was impaired. Collectively, the results indicate that HIF transcription factors are vital components of the mechanisms of follicular help during humoral responses.

Abstract B58: The role of EphA2 receptor tyrosine kinase in antitumor immunity mediated through programmed death ligand 2 (PD-L2) in non-small cell lung cancer (NSCLC)

Despite the advent of targeted therapies and breakthrough immunotherapies, lung cancer remains the number one cause of cancer-related death. Immune checkpoint inhibitors against PD-1 and PD-L1 induce remarkable and durable responses in a fraction of NSCLC patients, but most fail to respond or develop intolerable autoimmune-related toxicities. Additionally, the current standard of using PD-L1 expression alone as a biomarker is not strongly predictive of patient response and reveals the challenges that clinicians face when the understanding of the underlying biology lags behind clinical implementation.

EphA2 receptor tyrosine kinase is implicated in many solid tumors, including NSCLC, as well as inflammatory processes. Blockade of EphA2 induces apoptosis in NSCLC cells and increases survival in mouse models of NSCLC, demonstrating that EphA2 is a viable target for therapeutic intervention in NSCLC. Here, we examine how EphA2 in NSCLC cancer cells affects the PD-1 and PD-L1/2 checkpoint signaling axis in the tumor microenvironment. Preliminary studies suggest that EphA2 regulates the expression of PD-L2, but not PD-L1, in normal epithelium, as well as NSCLC cell lines. Knockdown of EphA2 by siRNA in BEAS-2B human bronchial epithelial cells shows decreased surface PD-L2 expression by flow cytometry. Additionally, PD-L2 expression is induced in human NSCLC cell lines by cytokine stimulation, specifically IFNg and IL-4. Knockdown of EphA2 in a subset of KRAS mutant human NSCLC cell lines after cytokine stimulation decreased PD-L2 expression. Analysis of NSCLC surgical specimens and survival data demonstrate that higher PD-L2 expression correlates with worse prognosis.

Despite PD-L2’s known role in immune tolerance, its impact on the tumor microenvironment is greatly understudied compared to PD-L1. Thus, further elucidation of EphA2s role in regulating PD-1 and PD-L2 interactions in functional assays and in in vivo mouse models will further our understanding of PD-1 and PD-L1/PD-L2 biology in the tumor immune microenvironment and will be critical for predicting the effects of immune checkpoint inhibitors.

Citation Format: Eileen F. Shiuan, Shan Wang, Ariel Raybuck, Mark Boothby, Jin Chen. The role of EphA2 receptor tyrosine kinase in antitumor immunity mediated through programmed death ligand 2 (PD-L2) in non-small cell lung cancer (NSCLC) [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr B58.

Hypoxia-Inducible Factors regulate metabolism and germinal center response

Germinal centers (GCs) are micronantomic structures that B cells proliferate, are selected for affinity maturation, undergo antibody (Ab) class-switch recombination and differentiate into Ab-secreting plasma cells and memory B cells. Both cell-autonomous and extrinsic metabolic reprogramming have emerged as modulators of T cellmediated immunity. We have shown that GC light zone (LZ) are hypoxic, and the low oxygen tension or persistent stabilization of Hypoxia-Inducible Factor (HIF)-1a and 2a alters B cell physiology and function. Depletion of HIF-1a and HIF-2a in CD4 T cells caused deficiencies of the antibody response due to HIF functions in both B lineage cells and the follicular CD4 subset. Among CD4 T cells, HIF restrained the FoxP3-positive T follicular regulatory (TFR) population while promoting CD40L expression. In the B lineage, HIF promoted humoral responses both through their capacity to promote effective proliferation and via enhancement of plasma cell differentiation. Collectively, these results provide evidence that HIF transcription factors regulate humoral responses via vital functions of B and Tfh cells. We propose that restriction of oxygen in lymphoid structure and organs, which can be altered in pathophysiological status, modulates humoral immune response and memory.

mTORC1 in B cells regulates antibody responses and promotes mitochondrial and metabolic fitness

B lymphocytes migrate among different micro-anatomic sites for diversification, selection, and eventual differentiation into antibody-secreting plasma cells. Emerging evidence supports the premise that aspects of the nutrient milieu vary within lymphoid micro-environments. However, the role of B cell-intrinsic metabolic programs in regulating B cell differentiation and antibody response quality remain unclear. We now show that the amino acid-sensing mTOR complex 1 (mTORC1) is essential for induction of Bcl6 and IRF4, key transcriptional regulators of germinal center and plasma cell fates. mTORC1 also enhances B cell proliferation upon exposure to antigen, increases the rate of somatic hyper-mutation, and is essential for generating high-affinity class switched antibodies. We also find that AMP-activated kinase (AMPK), an intracellular energy sensor, promotes plasma cell differentiation and antibody production. Collectively, these findings suggest mechanisms by which mTORC1 activity is critical for the germinal center reaction and producing class-switched, high-affinity antibodies, and indicate that AMPK can regulate the development and functional properties of terminally differentiated plasma cells.

Supported by NIH R01 AI113292, HL106812, and NCI T32CA009592-29

B cell-intrinsic and -extrinsic regulation of antibody responses by PARP14, an intracellular (ADP-ribosyl)transferase

The capacity to achieve sufficient concentrations of Ag-specific Ab of the appropriate isotypes is a critical component of immunity that requires efficient differentiation and interactions of Ag-specific B and Th cells along with dendritic cells. Numerous bacterial toxins catalyze mono(ADP-ribosyl)ation of mammalian proteins to influence cell physiology and adaptive immunity. However, little is known about biological functions of intracellular mammalian mono(ADP-ribosyl)transferases, such as any ability to regulate Ab responses. poly-(ADP-ribose) polymerase 14 (PARP14), an intracellular protein highly expressed in lymphoid cells, binds to STAT6 and encodes a catalytic domain with mammalian mono(ADP-ribosyl)transferase activity. In this article, we show that recall IgA as well as the STAT6-dependent IgE Ab responses are impaired in PARP14-deficient mice. Whereas PARP14 regulation of IgE involved a B cell-intrinsic process, the predominant impact on IgA was B cell extrinsic. Of note, PARP14 deficiency reduced the levels of Th17 cells and CD103⁺ DCs, which are implicated in IgA regulation. PARP14 enhanced the expression of RORα, Runx1, and Smad3 after T cell activation, and, importantly, its catalytic activity of PARP14 promoted Th17 differentiation. Collectively, the findings show that PARP14 influences the class distribution, affinity repertoire, and recall capacity of Ab responses in mice, as well as provide direct evidence of the requirement for protein mono-ADP-ribosylation in Th cell differentiation.

B cell intrinsic and extrinsic roles of ADP-ribosyltransferase, PARP14 in regulation of IgA and IgE responses (P1078)

Immunological memory, including both persistent and recall antibody (Ab) responses, enables more efficient and effective protection against infection, but key molecular mechanisms regulating emergence and persistence of memory B cells and plasma cells remain unclear. Poly-ADP-Ribose Polymerase14 (PARP14) is highly expressed in B cells and identified as a Stat6-interacting protein with intrinsic mono-ADP-ribosyltransferase (mART) activity. Although there are 17 PARP family members, the physiological function of most PARPs are not known well. Here we show that recall IgA and IgE production are impaired in PARP14-deficient mice. Further analysis revealed that, although PARP14 regulation of IgE involves B cell intrinsic function, IgA regulation by PARP14 was predominantly B cell extrinsic. Of note, PARP14 regulates the levels of CD103+ dendritic cells and T helper 17 (Th17) cells which might influence IgA generation in B cells. Moreover, PARP14 regulates the expression of RORα, Runx1 and Smad3 during Th17 differentiation, and mART activity of PARP plays a critical role in Th17 differentiation. Collectively, these results shed new light on differential mechanisms of Ig regulation, provide evidence that an endogenous mono ADP-ribosyltransferase regulates recall Ab production, and may have important implications for a new therapeutic target.