2.6.1. T-Cells
T-cells or T-lymphocytes are an integral part of adaptive immunity. They originate in the bone marrow but mature in the thymus, hence the name T-cells. T-cells are recognized from other lymphocytes due to their unique TCR displayed on the cell surface.
T-helper (T
h) cells (CD4
+ T-cells) express CD4 glycoprotein on their surface, recognize antigens presented by MHC class II and secrete cytokines that are necessary for both the cell-mediated and humoral immune response [
104]. Cytotoxic T-cells (CD8
+ T-cells) express CD8 glycoproteins, recognize antigens presented by MHC class I and eliminate virus-infected and tumor cells [
104]. Regulatory T-cells (Tregs) play an important role in maintaining balance by preventing the immune response to self-antigens and suppressing excessive immune response that can cause autoimmune diseases [
105]. T-helper 17 (Th17) cells are a unique CD4
+ T-cell subset characterized by the production of IL-17 that is a highly inflammatory cytokine playing an important role in the pathogenesis of several autoimmune diseases [
106].
CD4
+ T-cells differentiate into two major subgroups: Th1 and Th2 cells. Th1 cells mainly secrete the cytokines IFN-γ and IL-2, respond to intracellular microbes and stimulate phagocyte mediated uptake and elimination of microbes [
107,
108]. Th2 cells usually respond to extracellular pathogens such as gastrointestinal parasites, secrete mainly IL-4 and IL-5 cytokines and promote eosinophil activation and phagocyte-independent immune response [
107]. Cytotoxic T-lymphocyte-associated protein 4 (CTLA4), a naturally occurring protein receptor expressed on the surface of the T-cells, has the ability to inhibit the activation of the T-cell by competitively binding to CD80/86. CTLA4-Ig, a biologic that inhibits the full activation of T-cells, downregulates the Th2 response, and has little effect on Th1 response [
109]. Septic arthritis mice pretreated with CTLA4-Ig exhibited more severe joint inflammation but lower levels of IL-4 compared to the control mice [
110]. Although both CD4
+ and CD8
+ T cells are found in the inflamed synovium, CD4
+ T cells make up the overwhelming part. Furthermore, depletion of CD4
+ cells significantly ameliorates the course of septic arthritis in mice, whereas depletion of CD8
+ T cells does not alter the course of arthritis compared to control mice [
78]. Thus, it appears that CD4
+ T cells are pathogenic during
S. aureus septic arthritis due to their ability to produce proinflammatory cytokines such as TNF-α and IFN-γ via activated macrophages [
78]. Recent results also found that CD4
+ T cells promote the pathogenesis of
S. aureus pneumonia and
Pseudomonas aeruginosa septic arthritis [
111,
112].
The depletion of Tregs by anti-CD25 monoclonal antibodies aggravated the severity of septic arthritis without impact on bacterial clearance in mice [
79], suggesting the protective role of Tregs in development of septic arthritis. The role of Th17 cells in
S. aureus septic arthritis and sepsis is still unclear. However, IL-17 produced by Th17 cells is crucial for host defense against
S. aureus skin infections [
113] and septic arthritis [
114].
2.6.5. Cytokines
Several cytokines are secreted by cells of both the innate and the adaptive immunity and have different roles in S. aureus infections. Some of them will briefly be discussed below and are listed in .
Table 3. The role of cytokines in S. aureus septic arthritis and sepsis.
TNF-α, one of the most studied cytokines due to its role in inflammation and many diseases, is involved in the acute-phase reaction and is mainly secreted by activated macrophages and CD4+ cells, neutrophils, mast cells, and NK cells [
127].
TNF-α has a contrasting role during
S. aureus infections. In patients with
S. aureus arthritis, the levels of TNF-α have been shown to be highly elevated in the synovial fluid. Furthermore, it has been suggested that the levels of the cytokine could function as a predictor in determining the prognosis of the disease, with higher levels associated with worse prognosis [
128]. Animal studies have shown that TNF/lymphotoxin (LT)-α double knockout mice have significantly less severe
S. aureus arthritis compared to the wild-type mice [
118]. Indeed, we could also show that TNFR1 knockout mice exhibited less arthritis compared to wild-type mice in antibiotic-killed
S. aureus induced arthritis [
73]. Anti-TNF treatment was also able to abrogate arthritis induced by antibiotic-killed
S. aureus [
73]. Additionally, in the
S. aureus skin infection model, mice pretreated with anti-TNF agent exhibited smaller lesion (abscess) sizes compared to the control PBS-treated mice [
129]. On the other hand, the lack of TNF-α was associated with impaired ability of the host to successfully clear invading
S. aureus in the kidneys [
110,
118], thus leading to increased mortality [
118].
IL-1 cytokine family is a group of eleven cytokines that play an important role in the inflammatory response. Of these, most is known regarding IL-1α IL-1β, and IL-1 receptor antagonist (IL-1Ra). IL-1α plays a central role in the induction of fever, sepsis, and inflammation and is produced by activated macrophages, neutrophils, and endothelial and epithelial cells. IL-1β is predominantly produced by activated macrophages as a proprotein and is cleaved by caspase 1 into its active mature form [
130]. It plays an important role in pain, inflammation, and cartilage degradation in several inflammatory diseases [
131]. In
S. aureus systemic infections, IL-1R signaling is also essential to the host protection against the bacteria as shown by Hultgren et al. IL-1R
−/
− mice inoculated with
S. aureus developed significantly higher
S. aureus septic arthritis and sepsis compared to wild-type IL-1R
+/
+ mice [
119].
IFN-γ, a potent proinflammatory cytokine, is mainly produced by NK cells and T-cells. Apart from inhibiting viral and even bacterial infections, IFN-
γ activates and stimulates the macrophages to better phagocytose intracellular invaders. Different roles of IFN-
γ in
S. aureus triggered sepsis and septic arthritis have been described. Mice deficient of the IFN-γ receptor develop significantly more severe and frequent arthritis [
132]. The mortality levels due to sepsis are also significantly increased during the early stages of the infection in the mice lacking IFN-γ receptor, whereas in later stages the reverse is true with higher mortality levels in the wild-type mice [
132]. Likewise, in vivo administration of IFN-γ before and after inoculation of
S. aureus improved the survival of the mice while at the same time increased the severity and frequency of arthritis [
125]. The positive effects on mortality due to in vivo administration of IFN-
γ correlated with improved phagocytosis and better clearance of the bacteria in both, the liver and the kidneys. On the other hand, treatment of the mice with anti-IFN-γ monoclonal antibodies attenuated the severity and frequency of arthritis due to lower levels of serum TNF-α, IL-6, and IL-1β [
125].
IL-4 is an anti-inflammatory cytokine that has a role in differentiation of naïve T-cells into Th2 cells and the differentiation of B-cells into plasma cells. IL-4 promotes the cytotoxic activity of CD8+ cells, decreases the production of IFN-γ by T-cells and NK cells, and affects monocytes/macrophages by reducing their production of proinflammatory cytokines like IL-1, IL-6, and TNF-α [
133]. IL-4 inhibits the intracellular killing of
S. aureus in infected macrophages, without affecting phagocytosis and provides therefore a favorable milieu for survival of staphylococci [
122]. In
S. aureus infections, the dual role of IL-4 has been described depending on the genetic background of the host. In inbred C57BL/6 mice, IL-4 was shown to be a driving force of septic arthritis and sepsis by significantly impairing the capability of the host to clear the bacteria [
122]. Enhanced staphylococcal clearance from joints and kidneys, reduced mortality, and decreased frequency of arthritis was observed in IL-4 deficient C57BL/6 mice. However, in another inbred strain, 129SV mice, the opposite was true, i.e., IL-4 protected the mice from
S. aureus induced sepsis [
123]. IL-4 deficient 129SV mice had a thousand times higher bacterial growth in their kidneys, significantly elevated mortality, and delayed development of septic arthritis. A differential pattern of host responsiveness was seen between these mouse strains and explanation for the discrepant outcome could lie in the variation in circulating IL-4 levels—serum IL-4 was not detectable in C57BL/6 mice whereas increased IL-4 production was observed in 129SV mice in response to
S. aureus infection [
123].
Although IL-6 has been shown to have some anti-inflammatory features, it is usually regarded as a proinflammatory cytokine [
134]. Macrophages and T cells mainly produce IL-6 during infections or trauma. In
S. aureus infections, IL-6 is usually elevated together with other proinflammatory cytokines such as IL-1β and TNF-α [
128]. Synovial IL-6 together with synovial lactate and synovial fluid white blood cells count have been touted as good parameters for diagnosing septic arthritis [
120].
IL-10 is an anti-inflammatory cytokine produced mainly by monocytes and to a smaller extent the lymphocytes. IL-10 promotes Th2 response while downregulating Th1 cytokine secretion by macrophages and monocytes. IL-10 plays a crucial role protecting the host against
S. aureus septic arthritis by promoting bacterial clearance [
124].
IL-12 is primarily produced by monocytes, macrophages, and dendritic cells. Apart from stimulating the differentiation of naive T-cells to Th1 cells, IL-12 is also involved in the production of IFN-γ and TNF-α via T-cells and NK-cells. In
S. aureus infections, IL-12 is crucial for the survival of the host and deficiency of IL-12 is associated with significant accumulation of
S. aureus in many organs leading ultimately to the demise of the host [
121].
IL-17A is a proinflammatory cytokine produced by activated Th17 subset of T-cells. IL-17A plays a significant role in host defense against local
S. aureus infections due to its ability to induce chemokines that attract and recruit neutrophils [
135]. Thus, in local
S. aureus infection, IL-17A-/- mice developed more synovitis and erosions and more weight loss compared to the wild-type mice [
114]. On the other hand, IL-17A-/- mice did not differ from wild-type mice regarding the severity and the frequency of arthritis induced by antibiotic-killed
S. aureus [
73].