Inflammation plays a dominant role at all stages of tumor development: initiation, progression, and metastasis [
70]. Tumor-associated inflammation causes a decline in immune function and overrides tumor immunosurveillance and immunotherapy [
59]. Understanding the immune regulatory mechanisms of inflammation and balancing them in favor of tumor immunity will help improve cancer immunother-apy approaches. Studies in various immunopathological conditions highlight adaptive control of inflammation that need to be promoted in cancer.
The need for an adaptive control of inflammation becomes apparent as early as in neonatal mice and human newborns. As the embryonic development of the innate immune system precedes that of the adaptive system [
71], exposure to various forms of TLR stimulation following infection in neonatal mice causes a cytokine storm, which leads to high mortality [
72]. Similarly, in human newborns of small gestational age, excessive levels of TNF-
α, IL-1, and IL-6 have been detected following infections [
73–
76]. This is primarily due to a lack of control of the innate inflammatory response by the type I IFN- and IL-10-producing B cells [
77] and Tregs [
78], both of which are present at insufficient number in neonatal stage [
79,
80]. Tregs also coordinate the timing of entry of the innate immune cells into the infected tissue. Ablation of Treg cells delays the arrival of NK cells, DCs, and effector T cells to the site of herpes simplex virus infection in mice [
81]. Non-Treg resting T cells have also been shown to temper the production of IFN-
γ and TNF during initial innate inflam-mation in a hepatitis viral infection model. This is accomplished in an antigen-independent manner by direct contact inhibition, requiring the major histocompatibility complex molecule [
82]. These examples show that adaptive immune cells are indispensable for controlling inflammation.
Direct evidence of adaptive suppression of intracellular complexes called inflammasomes that process IL-1
β, a major pro-inflammatory cytokine [
83], came from a study in a murine peritonitis model [
84]. Activated T cells suppressed potentially damaging innate inflammation through inhibition of inflammasomes, including intracellular inflammation sensing proteins of the NLR family, NLRP1 and NLRP3. This blocked the caspase-1 axis and thereby decreased neutrophil recruitment. The T-cell-mediated contact-dependent blockade of macrophage caspase-1 activation, IL-1
β release, and IL-18 secretion in a cognate manner left the beneficial release of inflammatory mediators, such as chemokine (C-X-C motif) ligand 2 (CXCL2), IL-6, IL-12, and TNF (crucial for tissue healing) intact. TNF family ligands expressed by T cells were implicated in turning off the inflammasome. This is consistent with the established role of the TNF family in coordinating immune signaling networks [
85]. In addition, IFN-
γ produced by T cells in response to influenza infection has been shown to inhibit alveolar macrophages [
86]. Thus, T cells edit excessive innate inflammation by direct contact and secretion of cytokines, while maintaining their competence in antigen-specific recognition and stimulation. Induction of inflammatory response against
Plasmodium falciparum-infected erythrocytes and vaccine-induced cellular responses to rabies virus were also shown to involve crosstalk between T cells and NK cells [
87,
88]. In these studies, activation of NK cells and their IFN-
γ production and degranulation were crucially dependent on IL-2-mediated signals from CD4
+ T cells.
Chronic inflammation, which supports carcinogenesis, has also been found under the control of adaptive immune regulation. This has been particularly evident in adipose obesity, a physiological condition of chronic inflammation, that is initiated by alterations in the composition of T cells, B cells, and macrophages [
89–
92]. Infiltration of large numbers of effector CD8
+ T cells into epididymal adipose tissue, concomitant with a decrease in the numbers of CD4
+ T helper and Treg cells, is an early event during the development of mouse obesity [
93–
95]. In lean mice, Treg and Th2 cells dominate in the adipose tissue. These cells secrete IL-4 and IL-10, restricting inflammation in the resident adipose tissue macrophages. However, in obese mice, the accumulation of CD8
+ T cells and T
H1 cells in the adipose tissue signals the recruitment of inflammatory macrophages via chemokine (C–C motif) ligand (CCL) 2. Activated CD8
+ T cells secrete humoral factors known to induce macrophage migration, differentiation, and activation, including IFN-inducible protein-10, monocyte che-moattractant protein (MCP)-1, MCP-3, and RANTES (regulation upon activation, normal T cell expressed and secreted protein).
Thus, T cells contribute to the initiation and propagation of inflammation by directly affecting the production of inflammatory mediators such as IL-1, IL-6, TNF-
α, and serum amyloid A-3, as well as intercellular adhesion molecule-1 and matrix metalloproteinases (MMP) 2 and 3, leading to systemic insulin resistance and metabolic disorder [
93,
95]. In non-obese diabetic mice, CD8
+ T cells trigger nitric oxide production by macrophages, while macrophages trigger IFN-
γ production by CD8
+ T cells to cause islet destruction [
96]. T cells, largely via IFN-
γ, have also been shown to regulate the magnitude of the athero-genic proinflammatory response of macrophages [
97]. Deposited underneath the endothelium of arteries, the lipid-laden macrophages lead to the formation of inflammatory atherosclerotic plaques [
98]. Lipid-laden myeloid cells may play similar roles in causing inflammation, contributing to carcinogenesis. In a mouse model of human papilloma virus-driven squamous epithelium carcinogenesis, with the help of CD4
+ T cells, B cells have been shown to orchestrate macrophage-driven, tumor-promoting inflam-mation by producing antibodies that interact with and activate Fc
γ receptors on both tumor-resident and tumor-recruited myeloid cells at the tumor site [
99]. B cells can also drive M2-like polarization of macrophages and promote the growth of B16 melanomas [
100]. CD4
+ T cells can also regulate pulmonary metastasis of mammary tumors by enhancing pro-tumor properties of macrophages [
101]. Depending on the context, the cross-talk between B cells, T cells, and macrophages can mediate tumor-promoting inflammation or provide antitumor activity [
102]. Further identification of various cellular and molecular pathways that participate in cancer inflammation and the mechanistic correlation of T-cell activity with other leu-kocytes that may influence chronic inflammation, such as NK cells, Th17 cells, and B cells, or antigen presentation will be required to design strategies to check pro-tumor inflammation.
Increased numbers of T cells are also observed in the lungs of patients with chronic obstructive pulmonary disease. In a cigarette smoke-induced murine model of emphysema, a CD8
+ T-cell product, IFN
γ-inducible protein-10, was shown to induce the production of macrophage elastase (MMP-12) that degrades elastin and generates elastin fragments to serve as monocyte chemo-kines, augmenting macrophage-mediated lung destruction [
103]. The precise factors that cause the accumulation of activated CD8
+ T cells or Th1 cells in obese tissue or lungs are not known. Most likely, these factors are endogenous antigens and stimuli such as cholesterol crystals that develop in atherosclerotic plaques [
104] or smoke particles that accumulate in lungs. It is important to identify similar antigens expressed in early developing nascent tumors to strengthen tumor immunity.
These examples demonstrate that interdependent cooperative cross-talk between adaptive and innate immunity has the potential to minimize immunopathology and maximize host defense. Identification of the underlying signaling mechanisms responsible for the cross-talk between innate cells and the various populations of resting, effector, and regulatory T cells, as well as B cells, will help decipher new networks of immune regulation. This will reveal new intervention targets applicable for cancer therapy and prevention.