Neutrophils represent 50% to 70% of circulating leukocytes and are a fundamental component of innate immunity. It is essentially required for protection against microbes and is the first responder to foreign invaders [20]. Once a neutrophil recognizes a pathogen, it gets activated and phagocytoses the pathogen and produces reactive oxygen species (ROS) in a controlled fashion within the phagolysosome. Neutrophils also kill pathogens and eradicate them by non-oxidative mechanisms by secreting granules containing digestive proteases and perforins [21]. Neutrophils also recognized the pathogens and eliminated them by forming neutrophil extracellular traps (NETs). In sepsis, the maturation and functions of neutrophils are severely affected, leading to immune dysfunction and inflammation persistence [22]. Sepsis-induced, a state of delayed maturation of neutrophils, increased secretion of immunosuppressive mediators, reduced production of ROS, and impaired NETs formation that is associated with defective oxidative burst and microbial eradication [23]. The immune-suppressive functions of neutrophils during sepsis are shown in Table 1. Most of the patients who died from sepsis have pronounced derangement in neutrophils functions [24]. Several studies suggest that defects in neutrophil-mediated microbial containment and eradication could serve as therapeutic targets to improve the maturation, differentiation, and function of neutrophils.

The impact of sepsis on monocyte maturation, differentiation, and functions has been the subject of intensive investigations. The immunosuppressive functions of monocytes that occurred during sepsis are highlighted in Table 1. The reduced mononuclear cell HLA-DR expression and diminished capacity of monocytes to secrete pro-inflammatory cytokines in response to TLRs agonists are hallmark properties of sepsis [25]. Paradigm shifts in the secretion of pro-inflammatory cytokines to anti-inflammatory cytokines are significantly associated with the severity of sepsis, nosocomial infections, and poor outcomes [26]. A recent study shows the disparity in expression and function of monocytes subsets during sepsis [27]. These findings support the concept of monocytes reprogramming in sepsis. The mechanism responsible for endotoxin tolerances, diminished expression of HLA-DR, excessive production of immunosuppressive mediators, and decreased antigen presentation by monocytes during sepsis is not fully understood [25, 28]. Recent studies highlighted that epigenetic reprogramming in monocytes during sepsis is an epicentre of monocyte anergy [29]. However, the functional impact of these epigenetic alternations is not yet known.

MDSCs are a heterogeneous population of cells of myeloid origin that are differentiated into granulocytes, macrophages, and DCs [30]. Several studies conducted in the murine model of sepsis and humans showed a significantly elevated level of MDSCs in sepsis. The immunosuppressive functions of MDSCs during sepsis are shown in Table 1. In sepsis, these cells have been shown to block T cells proliferation and function [31]. These cells suppress the host adaptive immune response by inhibiting the production of IL-2 and interferon-γ (IFN-γ) by T cells. Due to the lack of universally accepted phenotypic markers, it is difficult to investigate the genesis of MDSCs and their functions in disease [32]. The role of MDSCs in the pathogenesis of sepsis and its association with disease severity are not fully understood.

DCs are professional APCs and are essentially required to regulate both arms of immunity: adaptive and innate immunity [33]. DCs recognize the exogenous antigens, process them, and present them to helper T cells for the activation and proliferation of T cells. DCs are required to recognize the pathogens and eradicate them through cell-mediated immunity [34]. In sepsis, the immunosuppressive functions of DCs are associated with increased sepsis-induced apoptosis of DCs and reduced surface expression of HLA-DR on DCs [35]. DCs secrete immune suppressive cytokines (IL-10) and help in expanding the Treg cells during sepsis [36]. All the immunosuppressive functions of DCs occurred in sepsis are shown in Table 1. Recent studies showed that preventing sepsis-induced DCs apoptosis or augmentation of DC function in the murine model of sepsis enhances sepsis survival [34]. The immune-suppressive function of DC has been ameliorated by treating models of sepsis with growth factor FMS-like tyrosine kinases three ligands (FLT3L). Treatment with FLT3L in models of burn sepsis improved DCs cytokine secretions and functions of T cells [37]. These observations suggested that improvement in DCs number and functions may be primary targets for therapeutic innervations in sepsis.

NK cells work as a rudimentary killer of cells infected by the viruses or lacked self-identifications [38]. NK cells are effector lymphocytes having cytotoxicity and cytokines secreting effector functions [39]. They protect the host from several microbial infections and tumors by limiting their spread and subsequent tissue damages. The functions of NK cells have not been fully understood in the consent of human disease due to their preferential location in tissues and low numbers in peripheral blood [40]. In sepsis, the levels of NK cells and their effector functions are markedly decreased. Like monocytes, NK cells become tolerant to TLR agonists in sepsis [41]. Impaired productions of cytokines in response to TLR agonist and cytotoxic functions have been reported in polymicrobial sepsis [42]. The molecular mechanism leading to the impaired cytotoxic function of NK cells, diminished productions of cytokines, and enhanced apoptosis of NK cells during sepsis are under meticulous investigation.

Generally, the innate immune cells recognize the invading pathogens and eliminate them, but sometimes the pathogen prevails, and the host response can become unbalanced and deleterious. The adaptive immune response is crucial for effective protection of the host against specific pathogenic microorganisms. Unlike the innate immune response, the adaptive immune response recognizes the pathogens too specifically, precisely differentiates self- and non-self-antigens, generates immunological memory against pathogens, and takes time to trigger the host immune response [43]. Ideally, the adaptive immune response recognizes and eliminates the infectious agent rapidly during re-infections with the same pathogen. The cross-talk between cell-mediated immune response (T-cells mediated) and humoral immune response (B cells mediated) immune response is required for effective protection of the host against specific pathogens [44].

About 15–30% of total leukocytes are T cells in human peripheral blood. The proportion of CD4⁺ Th cells and CD8⁺ cytotoxic T cells of total T cells accounts for 60–65% and 35–40%, respectively [45]. Th cells are a vital component of adaptive immunity which recognizes the pathogen through class II MHC and activates the B cells to secrete Abs and macrophages to destroy the ingested microbes [46]. The activation of CD4⁺ T cells results in polarisation into Th cells subsets Th1, Th2, Th17, and Treg cells. The difference among the subsets of Th cells is the cytokines and transcription factors that induce their differentiation and the cytokines they produce [47]. Th1 subset is essential for activating CD8⁺ T cells and for the formation of memory T cells via IL-2 secretion. The Th1-mediated cellular immunity is crucial to recognize the intracellular infection of a virus or bacteria and eliminate them by inducing phagocytosis or complement activation [48]. Th2 cells secrete IL-4, IL-5, and IL-13 cytokines upon activation and mediate class switching of B lymphocytes [49]. The Th2 cells mediated immunity is critical to protect against parasitic infections. Th17 cells are effector cells that secrete the cytokines IL-17 and IL-22 primarily in response to extracellular fungal and bacterial pathogens [50]. Treg cells are an essential subset of helper T cells that regulate the activated immune response once the pathogen is eradicated. Treg cells produce immunosuppressive cytokines [IL-10 and transforming growth factor (TGF)-β] and are essential in preventing autoimmunity [51].

The sepsis-induced apoptosis of CD4⁺ Th cells is the most notable cause of defective adaptive immunity. The presence of a significantly higher level of caspase 8 and caspase 9 in T cells of sepsis patients indicates that apoptosis occurs by intrinsic and extrinsic pathways [52]. Further, overexpression of pro-apoptotic proteins on T cells in sepsis patients was confirmed by several studies [53]. The apoptosis of T cells and increased expression of pro-apoptotic proteins contribute to defective adaptive immunity and immune suppression in sepsis. T-cell exhaustion and shift in polarity of Th cells from Th1 to Th2 are other significant causes of immune suppression in sepsis [54]. Marked reductions in the transcription factors and cytokines required for the polarisation and differentiation of Th cells subsets support the notion that CD4⁺ functions are suppressed during sepsis [50, 55]. The sepsis-induced immune suppression is relegated to Th1 and Th2 subsets and Th17, Treg cells, and other helper T cell subsets. The diminished cytokines secretion by Th17 cells has been reported in sepsis [56]. The level of Treg cells was reported significantly higher in sepsis patients as compared to non-sepsis patients. Further, levels of Treg cells were remained elevated in those patients who died due to sepsis. A recent study reported that the increased Treg cells and their ratio with Th17 (Treg/Th17) are associated with sequential organ failure (SOFA) score in post-traumatic sepsis patients [54]. Treg-associated immune dysfunctions and poor outcomes among trauma patients have been shown by several studies [57]. Recent research studies show that DCs secrete immune suppressive cytokines (IL-10 and TGF-β) and help in expanding the Treg cells during sepsis [36]. In an animal model of sepsis, it was shown that the adenosine was largely responsible to enhance the expression of forkhead box protein P3 (FoxP3, a major transcription factor of Treg differentiation) and leads to immunosuppression [58]. Treg-associated defective immune functions in sepsis have also been linked to increased solid tumor growth, probably from diminished T cells mediated effector functions and mononuclear cells effects [59]. Therefore, the notion exists that sepsis induces elevation of Treg cells that dysregulates the functions of the immune cells, contributes to poor outcomes, and may serve as the primary targets for immune modulation.

Although Treg cells can contribute to immune suppression in sepsis, Treg depletion in the murine models of sepsis does not improve mortality rates. In fact, the findings of one study in mice deficient in Treg cells showed that animals could not fight the initial sepsis infection [60]. Therefore, it is clear that a subtle balance of Treg cells is required to maintain immune homeostasis in sepsis.

CD8⁺ T cells are also called killer T cells or cytotoxic T cells that express CD8 glycoprotein on the cell membrane as their T cell receptor (TCR). The primary function of CD8⁺ T cells is to kill the virus-infected cells and tumor cells through cell lysis by degranulation or apoptosis [61]. CD8⁺ T cells recognize the antigenic peptide of pathogens through MHC class I and get activated. Upon activation, CD8⁺ T cells secrete cytokines, primarily tumor necrosis factor (TNF)-α and IFN-γ, and release cytotoxic granules to mediate the killing of virus-infected cells and tumor cells [62]. Several studies have confirmed the sepsis-induced apoptosis of CD8⁺ T cells [63]. The expression of caspases and pro-apoptotic proteins has been reported higher in CD8⁺ T cells of sepsis patients, especially in non-survivors compared to non-sepsis patients. The diminished production of cytokines and cytotoxic granules is the most notable cause of immune cell dysfunctions during sepsis.

γδ T cells relatively represent a deficient proportion of T cells. It is a distinct subset of T cells primarily present in the intestine and mucosal surface where they recognize the pathogens and mount an innate-like immune response by releasing cytokines such as IFN-γ, IL-17, and chemokines. γδ T cells exhibit innate and adaptive immunity features and play a pivotal role in host defense against invading pathogens, immune surveillance, and homeostasis [64]. The circulating level of γδ T cells was reduced in patients with sepsis. The reduced level of γδ T cells in sepsis patients is associated with sepsis severity and mortality rate [65, 66]. This is probably due to the apoptosis-induced loss of γδ T cells in the intestinal mucosa. The reduced level of γδ T cells and diminished production of cytokines by γδ T cells are primarily associated with immune defects during sepsis [65]. These findings show that preventing γδ T cells apoptosis and restoring the effector functions of γδ T cells could attenuate inflammatory responses during sepsis and prevent poor outcomes.

Like T cells, B cells also play an essential role in inducing innate and adaptive immune responses [67]. B cells could be differentiated into Abs secreting plasma cells and long-lasting memory cells upon activation. B cells also work as APCs and activate the effector T cells [68]. Several studies have reported the reduced circulating B cells and their function during sepsis [69]. Reduced production of Abs by B cells and the development of memory plasma B cells are associated with the severity of sepsis and mortality rate [70]. The relative proportion of B cells is reduced in sepsis patients, indicating that the apoptosis of B cells was induced. Depleting memory B cells contribute to the suppressed immune state experienced by septic patients. These observations confirm B cell dysfunction in sepsis.

PD-1 is upregulated on T cells in sepsis and cancer; it is often associated with T cells exhaustion, which results from prolonged exposure to antigens [83]. T cells exhaustion has been associated with immunosuppression during sepsis. The prolonged duration of sepsis and disease severity are often associated with high antigen loads and increased levels of anti-inflammatory cytokines [10]. A potential link between T cells exhaustion and upregulation of PD-1 on T cells is associated with sepsis’ morbidity and mortality [84]. Animal studies have shown that blockage of PD-1 improved survival in several clinical studies and restored T cells exhaustion in sepsis [85]. PD-1 ligand and PD ligand 1 (PD-L1) are expressed on monocytes, macrophages, DCs, epithelial cells, and endothelial cells. Anti-PD-1 and anti-PD-L1 treatments have shown positive responses and improved cancer survival in small clinical trials [83]. These data suggest that checkpoint inhibitors can be a promising treatment for sepsis. A recent study conducted in the model of sepsis that T cells obtained and treated with anti-PD-1 or anti-PD-L1 decreased apoptosis of T cells and enhanced T-cell function [86]. Altogether, these preclinical and clinical studies of several small trials of checkpoints inhibitors demonstrate significant restoration of T cells exhaustion. We speculate that anti-PD-1 and anti-PD-L1 might be advantageous in activating T cells immunity and reducing the apoptosis of T cells in sepsis. Cytotoxic T-lymphocyte antigen-4 (CTLA-4) is also known as CD152. CTLA-4 is one of the critical inhibitory regulators of early T cell activation and proliferation stages. CTLA-4 is a member of the immunoglobulin superfamily, which is expressed on the surface of T cells and transmits an inhibitory signal to CD4 and CD8 T cells. The inhibitory signal sent by CTLA-4 reduces T cells activation and proliferation [87]. Several small clinical trials were conducted in the animal models of cancer to block the inhibitory signal of CTLA-4 by Abs. Dose-dependent response of anti-CTLA-4 Ab therapy was reported in cancer; at the low dose, it improved the survival from cancer, but at high dose worsened the outcomes [88]. A recent study reported the positive response of anti-CTLA-4 therapy to the model of sepsis [85]. It reduced the sepsis-induced apoptosis of lymphocytes and restored T cells effector functions. However, anti-CTLA-4 therapy has a positive response in several small sepsis clinical trials but is not extensively investigated. Hence, further evaluation of checkpoints inhibitors in a large sepsis cohort will be required to assess the relative contribution of immune cell subsets to improved patient prognosis.