Why should lymphocytes immune profile matter in sepsis?

Abstract

The global incidence of critical illness has been steadily increasing, resulting in higher mortality rates thereby presenting substantial challenges for clinical management. Among these conditions, sepsis stands out as the leading cause of critical illness, underscoring the urgent need for continued research to enhance patient care and deepen our understanding of its complex pathophysiology. Lymphocytes play a pivotal role in both innate and adaptive immune responses, acting as key regulators of the balance between pro-inflammatory and anti-inflammatory processes to preserve immune homeostasis. In the context of sepsis, an impaired immunity has been associated with disrupted lymphocytic metabolic activity, persistent pro-inflammatory state, and subsequent immunosuppression. These disruptions not only impair pathogen clearance but also predispose patients to secondary infections and hinder recovery, highlighting the importance of targeting lymphocyte dysfunction in sepsis management. Moreover, studies have identified absolute lymphocyte counts and derived parameters as promising clinical biomarkers for prognostic assessment and therapeutic decision-making. In particular, neutrophil-to-lymphocyte ratio, and lymphopenia have gained recognition in the literature as a critical prognostic markers and therapeutic target in the management of sepsis. This review aims to elucidate the multifaceted role of lymphocytes in pathophysiology, with a focus on recent advancements in their use as biomarkers and key findings in this evolving field.

Key Words: Lymphocytes; Neutrophil-to-lymphocyte ratio; Sepsis; Septic shock; Chronic critical illness; Persistent inflammation, immunosuppression, and catabolism syndrome

Core Tip: Lymphocytes are essential effectors in the immune response to sepsis, contributing to both the initial defense against infection and the regulation of inflammation during the progression to chronic organ failure. One hallmark of sepsis is a profound disruption in lymphocyte homeostasis, that may lead to the development of an immunosuppressive state. Lymphocytes counts and derived variables show significant potential as prognostic markers in sepsis, offering insight into mortality risk and the likelihood of persistent organ dysfunction.

INTRODUCTION

The immune system is premised on defending organisms from invaders and promoting healing of injuries. Its responses are broadly categorized into two mechanisms: Innate and adaptive immunity[1,2]. The innate response corresponds to the initial reaction, in which cells originating from the myeloid progenitor differentiate to halt the spread of harmful agents. Polymorphonuclear cells and macrophages play a central role in this phase, eliminating pathogens through phagocytosis[3]. Specialized innate immune cells, such as dendritic cells, further enhance the immune response by transforming into antigen-presenting cells after phagocytizing the antigen. These cells bridge innate and adaptive immunity by presenting antigens to lymphocytes, thereby initiating the adaptive immune response[1,3]. This enables lymphocytes to perform immune recognition, thereby distinguishing between inert endogenous components and harmful external agents and memorizing antigen patterns[1,2].

This review addresses the role of lymphocytes counts and derived parameters in the immune response, with a particular focus on their involvement in sepsis. Additionally, it discusses lymphocytes as clinical biomarkers and their emerging potential as therapeutic targets in sepsis management. Lymphocytes constitute approximately 40% of the white blood cell population and include B cells, T cells, and natural killer cells and their subtypes (Figure 1). B cells and T cells are critical components of the adaptive immune response system, mediating responses to bacterial and viral pathogens, through antibody production and cell-mediated immunity, respectively[4]. In the context of sepsis, lymphocytes play a pivotal role in orchestrating the host immune response during both the acute and chronic phases of the syndrome[1]. Importantly, the lymphocytic response to sepsis may present distinct phenotypic profiles that are often associated with different clinical outcomes[4]. These profiles are influenced by the stage of the immune response - whether early or late - and are characterized by differences in lymphocyte proliferation, the expression of immunity-related proteins, and the secretion of signaling molecules. These cellular phenotypic transitions depend on metabolic plasticity to meet the substantial energy demands associated with immune activation, proliferation, and effector functions[2].

Figure 1 Schematic view of the main subtypes of lymphocytes involved in sepsis. Th: T-helper; IFN: Interferon; TNF: Tumor necrosis factor; IL: Interleukin; Treg: Regulatory T-cell; TGF: Transforming growth factor; APC: Antigen presenting cells; Ig: Immunoglobulin; NK: Natural killer.

HOW LYMPHOCYTES RESPOND DURING SEPSIS

Sepsis is a global health priority[5] and is associated with a high mortality rate, even in middle-income countries[6] and high-income countries[7]. Despite different definitions over the last few decades, sepsis is best defined as a syndrome shaped by pathogens and host factors in a dysregulated systemic host response[8]. It is regulated by inflammatory effectors such as tumor necrosis factor alpha, interleukin (IL)-6, and IL-1, which modulate cellular responses and influence patients’ clinical outcomes[9]. Therefore, sepsis can potentially interfere with the host immune system and result in drastic changes[10]. During the first 24 hours - 48 hours following sepsis onset, the immune response is dominated by a pro-inflammatory burst, characterized by robust activation of cytokine pathways. Beyond this acute phase, the immune system undergoes a shift toward an anti-inflammatory state, often described as compensatory or maladaptive[11,12]. This transition can compromise immune surveillance and contribute to immune dysfunction.

One hallmark of sepsis is a profound disruption in lymphocyte homeostasis, characterized by a significant reduction in lymphocyte counts, termed sepsis-induced lymphopenia. This may lead to the development of an immunosuppressive state, predisposing individual patients to secondary infections induced by low pathogenicity invaders thereby exacerbating patient morbidity and delaying recovery[9]. This depletion affects CD4⁺ T cells and CD8⁺ T cells, B cells, and natural killer cells, resulting in impaired lymphocyte functionality and contributing to immune paralysis. The duration of immunoparalysis needs to be hampered since it contributes to increased morbidity associated with sepsis[11,13]. Therefore, lymphocytes phenotype in sepsis may transit through states of acute response, persistent pro-inflammatory signaling to immunosuppression. Both quantitative and qualitative disturbances in lymphocytes have been observed across these states[14]. Indeed, a reduced lymphocyte count combined with metabolic dysregulation can further disrupt the crosstalk between the adaptive and innate immune systems, undermining the coordinated response required to resolve injury effectively[15,16].

ARE LYMPHOCYTES ASSOCIATED WITH THE PROGNOSIS OF SEPSIS?

The use of inflammatory biomarkers for prognostic and therapeutic assessments in sepsis has gained significant attention. Biomarkers such as C-reactive protein, procalcitonin, IL-1β, and IL-6 are widely studied but not always accessible in clinical practice, particularly in middle- and low-income countries. This underscores the requirement of inexpensive, easy-to-perform, and reproducible biomarkers to aid clinical decision-making in sepsis management[17,18]. Lymphocyte-derived variables have emerged as promising biomarkers in recent years, providing insights into both acute injury and the progression of chronic sepsis. These variables can be derived from routine peripheral blood samples, and include total lymphocyte counts and ratios of lymphocytes to other immune cells (e.g., neutrophils and monocytes) or immune-related molecules (e.g., albumin and high-density lipoprotein). For instance, lymphopenia - defined as an absolute circulating lymphocyte count < 1000 cells/mm³ -has been associated with worse prognosis in sepsis, with derived ratios like the neutrophil-to-lymphocyte ratio (NLR) and monocyte-to-lymphocyte ratio (MLR) providing additional read-out of patients with worse prognosis[4,19].

Several studies in different countries and subgroups of intensive care unit (ICU) patients have demonstrated strong associations between lymphopenia, its derived markers, and unfavorable clinical outcomes (Table 1)[20-32]. In a large Taiwanese database, it was found that an early lymphopenia was linked to increased one-year mortality in critically ill surgical patients, with this association remaining robust after advanced statistical adjustments such as propensity score matching[33]. Similarly, elevated NLR levels have been correlated with higher mortality in ICU patients[34], and the persistence of lymphopenia has been tied to worse outcomes, including increased risk of nosocomial infections, acute kidney injury, and 28-day mortality[35,36]. Both the monocyte-lymphocyte ratio and NLR showed accurate performance as biomarkers[35]. The prognostic value of lymphocyte counts and ratios extends beyond traditional ICU settings. For instance, in the context of corona virus infectious disease-2019, elevated NLR has been predictive of disease severity, prolonged ICU stays, and mortality[37,38]. Other ratios, such as the platelet-to-lymphocyte ratio and lymphocyte-to-C-reactive protein ratio, have shown promise as moderate predictors of septic shock progression during prolonged ICU stays[39-41].

Table 1 Observational studies exploring lymphocyte-derivates variables and clinical outcomes in sepsis.

Ref.	Population	Variable measured	Outcome
Biyikli et al[20]	Adult patients older than 65 years with sepsis or septic shock in emergency admission	Platelet-lymphocyte ratio	Platelet-lymphocyte ratio was not associated with 30-day mortality (207.6 in non-survivors vs 168.3 in survivors)
Djordjevic et al[21]	Critically ill injured patients admitted to surgical ICU	PLR, MLR, NLR	There was no difference in the biomarkers regarding hospital mortality in septic trauma patients (8.5 in non-survivors vs 9.6 in survivors)
Gharebaghi et al[22]	Critically-ill patients with sepsis due to gram-negative pathogens	Neutrophil-lymphocyte ratio	Patients who deceased had increased values of NLR in day 2 (14.9 vs 9.3) and in day 3 of ICU admission (17.2 vs 9.1), but not at day 1 (13 vs 9.8)
Goda et al[23]	Neurosurgical critically ill patients with cathteter-associated urinary tract infections or central line-associated bloodstream infections	Neutrophil-lymphocyte ratio	An increased NLR was an independent predictor of in-hospital mortality in central-line associated bloodstream infections (7.29 in non-survivors vs 4.46 in survivors)
Guo et al[24]	Critically ill patients with sepsis, from MIMIC-IV database	Neutrophil + monocyte/lymphocyte ratio	An increased NMLR is associated with increased 30-day mortality (12.24 non-survivors vs 8.71 in survivors)
Hsu et al[25]	Critically ill cirrhotic patients with septic shock	LMR and NLR	Non-survivors had increased NLR (13 vs 10.3) and decreased LMR (1.1 vs 2.3) when compared with survivors
Li et al[26]	Critically ill septic shock patients	NLR	NLR at day 3 and delta NLR (day 3 - day 1), but not NLR at day 1 were associated with 28-day mortality, in univariate and multivariate analysis
Liang et al[27]	Critically ill patients with bloodstream infections	NLR	Delta NLR (NLR 48 hours - NLR at 0 hour) were higher in patients with shock
Liu et al[28]	Critically ill patients with sepsis, from MIMIC-IV database	LHR	Low values of LHR were associated with 90-day mortality
Lorente et al[29]	Critically ill patients with sepsis	NLR	Increase in NLR at day 1, day 4 and day 8 were associated with 30-day mortality, when controlled for SOFA score and lactate at this time intervals
Sari et al[30]	Critically ill patients with sepsis	NLR	NLR at day 1 of sepsis is not associated with ICU mortality. At day 3, NLR greater than 15 is strongly associated with mortality
Wu and Qin[31]	Critically ill patients with sepsis	NLR, PLR, MLR	There was no difference between variables measured at baseline in survivors and non-survivors at 28-days post ICU admission
Xiao et al[32]	Adult septic patients from MIMIC-IV database	N/LP	High and middle terciles of N/LP at baseline were associated with an increase in the incidence of septic AKI (HR 1.3 and 1.2, respectively), as compared with the lower tercile

ICU: Intensive care unit; PLR: Platelet-to-lymphocyte ratio; MLR: Monocyte-to-lymphocyte ratio; NLR: Neutrophil-to-lymphocyte ratio; AKI: Acute kidney injury; HR: Hazard rate; LHR: Lymphocyte-to-high-density lipoprotein ratio; LMR: Lymphocyte-to-monocyte ratio; MIMIC: Medical information mart for intensive care; N/LP: Neutrophil to lymphocytes and platelets ratio; SOFA: Sequential organ failure assessment; NMLR: Neutrophil + monocyte/lymphocyte ratio.

Sepsis-induced lymphopenia involves apoptotic mechanisms mediated by death receptor activation and mitochondrial pathways in lymphocytes[39-41]. This reduction is particularly significant in CD8⁺ T cells, with survivors of septic shock showing recovery of these cell counts after five days post-ICU admission[42]. Decreases in T-cell subpopulations (CD3⁺, CD4⁺, and CD8⁺) and B cells have been associated with an increased risk of nosocomial infections and diminished immunoglobulin M production[43,44]. However, the functional implications of lymphocyte subclasses remain underexplored, necessitating further investigation to refine their prognostic value and therapeutic target potential.

The impact of lymphopenia varies across the sepsis timeline (Figure 2). At sepsis onset, lymphopenia may not always correlate with 28-day mortality, but resolution of lymphopenia by day 4 has been linked to improved outcomes, even after adjusting for confounders[45]. Conversely, persistent lymphopenia from ICU admission to day 3 significantly increases the risk of secondary infections and 28-day mortality[46]. These findings suggest that lymphocyte trajectories during critical illness, particularly persistent lymphopenia, may serve as an accurate biomarker for short-term mortality, chronic critical illness development, and sepsis progression to septic shock[26,47]. When compared with the non-lymphopenic group, patients with sepsis and lymphopenia more frequently required ICU admission, had a longer hospital length of stay, and presented with a higher rate of in-hospital, and 30-day mortality[35,46,48]. Despite these promising findings, the clinical applicability of lymphocyte-derived biomarkers remains limited by inconsistencies in cutoff definitions for persistent lymphopenia, which range from 760 cells/μL to 1000 cells/μL across studies[35]. Additionally, while lymphopenia is associated with 90-day mortality and rehospitalization, its predictive accuracy for these outcomes is modest[49]. Larger, multicenter studies are needed to validate these findings and explore the implications of lymphopenia on clinically meaningful outcomes beyond mortality, including functional recovery and quality of life in different patient subgroups.

Figure 2 Evolution of lymphocyte count in sepsis. MOF: Multiorganic failure; PIICS: Persistent inflammation, immunosuppression and catabolism syndrome.

ARE LYMPHOCYTES ASSOCIATED WITH CRITICAL CHRONIC ILLNESS?

Sepsis remains a leading cause of mortality, with most deaths occurring within 72 hours of diagnosis, underscoring the importance of early recognition and intervention to improve survival rates[11]. However, some patients progress to a state of persistent critical illness, characterized by worsening or unresolved multiple organ failure and a heightened risk of unfavorable outcomes. Even after surviving the acute phase of sepsis, patients often endure prolonged ICU stays and remain vulnerable to secondary infections. This condition, now termed persistent inflammation, immunosuppression, and catabolism syndrome (PIICS), is associated with substantial clinical, economic, and social burdens[50]. PIICS manifests as long-term dysfunction across multiple systems, including neurocognitive, muscular, respiratory, renal, and cardiovascular functions, often leading to lasting functional impairment. A defining feature of immunosuppression in PIICS is lymphopenia, specifically a lymphocyte count below 800 cells/μL. During sepsis, a compensatory anti-inflammatory response mediated by cytokines such as IL-10 and transforming growth factor beta counterbalances the initial pro-inflammatory cascade driven by IL-6 and IL-1[51]. While this response may mitigate tissue damage, it can also induce T-cell exhaustion and expand regulatory T-cell (Treg) populations, a subset of immunomodulatory CD4⁺ T cells that suppress immune responses to control inflammation[52,53]. Tregs play a dual role in perpetuating persistent inflammation and immunosuppression, impairing the host’s ability to resolve infections or respond to new threats[53,54].

Interestingly, the dysregulated expression of IL-6, IL-1, and IL-10 in sepsis has been shown to interfere with lymphocyte metabolism[55]. Persistent inflammation further disrupts mitochondrial function diminishing bioenergetic capacity and exacerbating immunoparalysis[56]. The mitochondrial bioenergetic dysfunction impairs adenosine triphosphate support in the presence of a hypercatabolic state, thereby undermining the appropriate immune function[56]. This imbalance sustains a vicious cycle of unresolved inflammation, hypercatabolism, and progressive lymphocyte-mediated immunosuppression or immune exhaustion. Sepsis-induced immunoparalysis involves multiple mechanisms within the adaptive immune response, including lymphocyte apoptosis, diminished antigen presentation, impaired antigen-driven proliferation, increased suppressive Treg populations, and T-cell exhaustion[11]. While studies recognized that both lymphopenia and reduced T-cell functionality have been strongly associated with adverse outcomes[10] the precise relationship between these variables requires further investigation (Figure 3). Nowadays, lymphopenia is a hallmark of chronic critical illness[57], especially when it occurs alongside PIICS[58]. This persistent immunosuppressive state contributes to prolonged disease courses and underscores the need for targeted interventions to restore lymphocyte function and mitigate long-term complications in sepsis survivors.

Figure 3 Interplay between inflammatory response, quantitative and qualitative impairment in lymphocytic response in sepsis. TNF: Tumoral necrosis factor; IL: Interleukin; PIICS: Persistent inflammation, immunosuppression and catabolism syndrome.

ARE LYMPHOCYTES POTENTIAL THERAPEUTIC TARGETS?

Recent findings have highlighted immunostimulatory therapies as novel strategies to restore host defense in sepsis and prevent opportunistic infections[59]. Among these, IL-7 stands out as a promising candidate for addressing sepsis-induced T-lymphocyte immune dysfunction. IL-7 has demonstrated the ability to enhance T-cell survival and functionality, exhibiting anti-apoptotic properties, promoting robust proliferation of CD4⁺ T cells and CD8⁺ T cells, and improving cytokine production. However, its effects are primarily limited to naïve and memory T lymphocytes rather than effector-activated T cells[59].

To date, two clinical trials have specifically assessed recombinant human IL-7 in septic patients with lymphopenia. In a phase II randomized controlled trial, Francois et al[60] observed a 3-fold to 4-fold sustained increase in lymphocyte counts, including a rise in circulating CD4⁺ T cells and CD8⁺ T cells, without inducing cytokine storms, exacerbating inflammation, or causing organ dysfunction. A more recent trial also reported significant increases in absolute lymphocyte counts (including both CD4⁺ and CD8⁺ subsets) with IL-7 administration compared to placebo[14]. Both studies confirmed the safety profile of IL-7, though intravenous administration was associated with transient fever and respiratory distress, adverse effects not observed with intramuscular administration[14]. It is worth noting that these trials were not designed to evaluate key clinical outcomes such as mortality, length of hospital stay, or days free from organ support. Consequently, the long-term benefits of IL-7, including its impact on reinfections, rehospitalizations, and non-infectious complications, remain uncertain. Further research is needed to determine the specific patient populations most likely to benefit, such as those with lymphopenic sepsis at admission vs patients with persistent lymphopenia during their clinical course.

CONCLUSION

Lymphocytes play a central role in the host’s response to sepsis, both in the initial immune activation and in the progression to chronic organ failure. Lymphocyte counts and their derived variables hold significant promise as prognostic biomarkers in septic patients. However, large-scale, multicenter studies are essential to validate their clinical utility and to explore the therapeutic potential of lymphocyte-targeting interventions such as IL-7. These efforts will be critical in refining strategies to improve the management and outcomes of patients with sepsis.