Metabolism-associated danger signal-induced immune response and reverse immune checkpoint-activated CD40+ monocyte differentiation

Abstract

Adaptive immunity is critical for disease progression and modulates T cell (TC) and antigen-presenting cell (APC) functions. Three signals were initially proposed for adaptive immune activation: signal 1 antigen recognition, signal 2 co-stimulation or co-inhibition, and signal 3 cytokine stimulation. In this article, we propose to term signal 2 as an immune checkpoint, which describes interactions of paired molecules leading to stimulation (stimulatory immune checkpoint) or inhibition (inhibitory immune checkpoint) of an immune response. We classify immune checkpoint into two categories: one-way immune checkpoint for forward signaling towards TC only, and two-way immune checkpoint for both forward and reverse signaling towards TC and APC, respectively. Recently, we and others provided evidence suggesting that metabolic risk factors (RF) activate innate and adaptive immunity, involving the induction of immune checkpoint molecules. We summarize these findings and suggest a novel theory, metabolism-associated danger signal (MADS) recognition, by which metabolic RF activate innate and adaptive immunity. We emphasize that MADS activates the reverse immune checkpoint which leads to APC inflammation in innate and adaptive immunity. Our recent evidence is shown that metabolic RF, such as uremic toxin or hyperhomocysteinemia, induced immune checkpoint molecule CD40 expression in monocytes (MC) and elevated serum soluble CD40 ligand (sCD40L) resulting in CD40⁺ MC differentiation. We propose that CD40⁺ MC is a novel pro-inflammatory MC subset and a reliable biomarker for chronic kidney disease severity. We summarize that CD40:CD40L immune checkpoint can induce TC and APC activation via forward stimulatory, reverse stimulatory, and TC contact-independent immune checkpoints. Finally, we modeled metabolic RF-induced two-way stimulatory immune checkpoint amplification and discussed potential signaling pathways including AP-1, NF-κB, NFAT, STAT, and DNA methylation and their contribution to systemic and tissue inflammation.

Keywords: CD40+ MC; Immune checkpoint; Metabolic risk factors; Metabolism-associated danger signal; Reverse-immune checkpoint.

Fig. 1

Innate immunity. a Innate immunity and novel MADS recognition. The classical innate immune system provides immediate and non-specific defense against pathogen or injury-generated molecules via PAMP/DAMP+PRR recognition in phagocytes and TC. Super Ag, a subset of pathogen toxins, can also bind to a multitude of TCR leading to TC activation. In addition, we propose a novel MADS recognition pathway, which allows metabolic risk factors to activate the innate immunity via responsive metabolic sensors in phagocytes and TC. The activation of innate immunity leads to pathogen elimination and inflammation (APC formation, cytokine generation, and TC activation). b Evidences of innate immunity in TC. Stimuli such as PAMP/DAMP, inflammatory cytokines and super Ag activate different subsets of TC and stimulate TC proliferation, inflammatory cytokine production, and phagocytosis. Words in red emphasize our newly proposed recognition pattern. Abbreviations: APC antigen present cell; Ag antigen; Ab antibody; BC B cell; BCR B cell receptor; CpG C, a cytosine triphosphate deoxynucleotide; p phosphodiester; G a guanine triphosphate deoxynucleotide; CTL cytotoxic T lymphocytes; DAMP danger-associated molecular patterns; d days; Foxp3 forkhead box P3; h hours; IL interleukin; IFN interferon; LPS lipopolysaccharide; MHC major histocompatibility complex; MADS metabolism-associated danger signal; NLR NOD (nucleotide-binding and oligomerization domain)-like receptors; PAMP pathogen-associated molecular patterns; PRR pattern recognition receptor; Poly(I:C) polyinosinic-polycytidylic acid; Pam ₃ CSK ₄ tripalmitoyl-S-glycero-Cys-(Lys)4; RF risk factor; R848 Imidazoquinoline Resiquimod; SEB staphylococcal enterotoxin B; TC T cell; TCR T cell receptor; Th17 T helper 17 cell; TLR Toll-like receptors; SPA staphylococcal protein A; TNF tumor necrosis factor; TGF-β transforming growth factor beta

Fig. 2

Adaptive immunity with novel signal 4, the metabolic RF recognition. The adaptive immunity is characterized by Ag specificity and immunologic memory leading to TC and BC activation. There are two types of adaptive immunity: TC immunity (cell-mediated immunity) and BC immunity (humoral immunity). Classically, each involves three activating signals. We propose a novel signal 4 (metabolic RF recognition) mediated by metabolic sensor. a TC immunity. TC activation involves four distinct signals. In signal 1 (Ag recognition), the Ag peptide is presented by MHC on the APC to Ag-specific TCR on TC. Signal 2 (immune checkpoints) involves ligand and receptor binding on APC and TC. Signal 3 responds to inflammatory cytokine stimulation. The novel signal 4 describes metabolic RF using a metabolic sensor leading to MC (APC) differentiation, inflammatory cytokine production, and the enhancement of signals 2 and 3. b BC immunity. BC activation involves Ag binding to BCR (signal 1), ligand and receptor binding (signal 2), cytokine stimulation (signal 3), and metabolic RF recognition (signal 4). Words in red emphasize our newly proposed signal. Abbreviations: APC antigen present cell, Ag antigen, BC B cell, BCR B cell receptor, RF risk factor, HHcy hyperhomocysteinemia, MHC major histocompatibility complex, MC monocyte, sCD40L soluble CD40 ligand

Fig. 3

Immune checkpoint and its characterizations. a Immune checkpoint classification. Immune checkpoints are classified as one-way immune checkpoint and two-way immune checkpoint based on signal 2 direction and are further divided into stimulatory and inhibitory immune checkpoints. b. One-way immune checkpoint. The one-way immune checkpoint only involves forward signaling in signal 2. The ligation of co-stimulatory pair molecules triggers forward stimulatory immune checkpoint inducing TC proliferation, whereas the inhibitory immune checkpoint induces TC suppression or death. c Two-way immune checkpoint. The two-way immune checkpoint involves both forward and reverse stimulatory signaling. The reverse stimulatory immune checkpoint induces either TC proliferation or MC (APC) differentiation/inflammation. The inhibitory immune checkpoint leads to TC suppression/death or APC death. Words in red emphasize our newly proposed signal. Abbreviations: APC antigen present cell, MC monocyte, RF risk factor, sCD40L soluble CD40 ligand, TC T cell

Fig. 4

CD40:CD40L stimulatory immune checkpoint (molecular mechanism and biological function). a Forward immune checkpoint. CD40:CD40L stimulation occurs when B7 engages CD28. In TC, CD40:CD40L ligation, via ZAP-70 activation, leads to activating three important signal pathways (MAPK/NF-κB/calcineurin) and promotes gene transactivation and TC activation. b Reverse-immune checkpoint. In APC, CD40:CD40L ligation, via TRAF2/3/5/6 activation and the following STATS, NF-κB, and AP-1 activation, promotes gene expression and APC inflammation. c TC contact-independent immune checkpoint. Metabolic RF increases circulating sCD40L and CD40 in MC. sCD40L:CD40 co-stimulation results in CD40 MC differentiation and inflammation via metabolic sensor and DNA hypomethylation-related mechanisms. Words in red emphasize our new findings and proposed signal. Abbreviations: APC antigen present cell, AP-1 activator protein 1, BC B cell, ERK extracellular signal-regulated kinase, HHcy hyperhomocysteinemia, Ig immunoglobulin, IKK I-κB kinase, I-κBs I-κB proteins, JNK JUN amino-terminal kinase, LAT linker for activation of T cells, MAPK mitogen-activated protein kinase, MØ macrophage, MC monocyte, NKC natural killer cell, NF-κB nuclear factor κB, p phosphorylated, PLCγ1 phospholipase C gamma 1, sCD40L soluble CD40 ligand, SMC smooth muscle cell, STAT3 signal transducers and activator of transcription-3, TC T cell, Treg regulatory T cell, TRAF tumor necrosis factor receptor, ZAP70 zeta chain-associated protein kinase

Fig. 5

CD40⁺ MC is a reliable biomarker for CKD severity. CD40⁺ MC and plasma sCD40L were examined in human CKD subjects (experimental details in Yang et al. [15]). Peripheral WBC were isolated after red blood cell lysis and stained with antibodies against CD14, CD16, and CD40 for flow cytometry analysis. a CD40⁺ MC. CD40⁺ MC subset was elevated in patients with CVD and CVD+CKD compared to healthy subjects and increased with CKD severity. b Plasma sCD40L. sCD40L was elevated in patients with CVD and CVD+CKD compared to healthy subjects. c CD40⁺ intermediate MC. CD40⁺ intermediated MC subset was elevated in patients with CVD and CVD+CKD compared to healthy subjects and increased with CKD severity. d Intermediate MC. Intermediate MC subset was elevated in CVD patients, but not further increased in CVD+CKD patients. *p < 0.05 vs healthy. Abbreviations: CKD chronic kidney disease, CVD cardiovascular disease, MC monocyte, PBMC peripheral blood mononuclear cells, sCD40L soluble CD40 ligand, WBC white blood cells

Fig. 6

Working model of metabolic risk factor-induced two-way stimulatory immune checkpoint amplification and systemic/tissue inflammation. Metabolic risk factors, such as HHcy, uremic toxins, and other RF,, stimulate two-way stimulatory immune checkpoint amplification in TC, APC (MC), and possibly in PL via MADS recognition. In response to metabolic RF stimulation, metabolic sensors mediate TC activation via MAPK/NF-κB/calcineurin pathway, APC inflammation via STAT3MAPK/NF-κB pathway, MC differentiation via DNA hypomethylation, and possibly sCD40L production in PL via MAPK/NF-κB activation. TC activation and APC inflammation finally result in inflammatory cytokine production and systemic/tissue inflammation. Words in red emphasize our newly proposed signal pathway. Abbreviation: APC antigen present cell, HHcy hyperhomocysteinemia, MC monocyte, MAPK mitogen-activated protein kinase, MADS metabolism-associated danger signal, NF-κB nuclear factor κB, RF risk factor, PL platelet, STAT3 signal transducers and activator of transcription-3, sCD40L soluble CD40 ligand, TC T cell