Pattern of Serum Cytokine Expression and T-Cell Subsets in Sickle Cell Disease Patients in Vaso-Occlusive Crisis

The pathogenesis of sickle vaso-occlusive crisis (VOC) in sickle cell disease (SCD) patients involves the accumulation of rigid sickle cells and the stimulation of an ongoing inflammatory response, as well as the stress of infections. The immune response, via cytokine imbalances and deregulated T-cell subsets, also has been proposed to contribute to the development of VOC. In this study, a panel of high-sensitivity cytokine kits was used to investigate cytokines in the sera of SCD patients in VOC. The results were compared primarily with those for stable SCD patients and secondarily with those for normal healthy people who served as controls. The cytokines studied included interleukin-2 (IL-2), IL-4, and IL-10. Lymphocyte subsets of patients with VOC were also studied and were compared with those of both control groups (20 stable patients without crisis SCD group] and 20 normal healthy controls NHC]). The VOC group was notable for remarkably elevated levels of IL-4, among the three cytokines tested, compared with those for the SCD and NHC groups. Patients with VOC also differed from stable SCD patients and NHC by having notably lower IL-10 levels, as well as the lowest ratio of CD4⁺ to CD8⁺ T cells (0.7). The patterns of the proinflammatory cytokine IL-2 did not differ between VOC and stable SCD patients, but NHC had significantly lower IL-2 levels than both the VOC and SCD groups. Our results demonstrate coexisting levels, both high and low, of TH1- and TH2-type cytokines, as well as diminished levels of T-cell subsets in VOC. These results are discussed in an effort to better understand the importance of the immune system profile in the pathogenesis of sickle cell VOC. Since the possibility that a cytokine imbalance is implicated in the pathogenesis of sickle cell crisis has been raised, our results should prompt further investigation of the host immune response in terms of TH1 and TH2 balance in sickle cell crisis.Sickle cell disease (SCD) is a chronic, incurable condition presenting primarily as anemia (sickle cell anemia SCA]) in people homozygous for hemoglobin S (HbS). This abnormal hemoglobin, resulting from the replacement of glutamic acid at position 6 of the β-globin chain by valine, is responsible for erythrocyte distortion and fragility in these patients, as well as for thrombosis, fever, splenomegaly, joint pain, lethargy, and weakness. Sickle cell crises refer to the sudden attacks of pain, at various levels of severity, that occur during the lifetime of the patient with sickle cell disease (1, 3). Of these, the painful vaso-occlusive crisis (VOC) is the most common and is characterized by fever, leukocytosis, joint effusions, and tenderness, which occur in about 50% of patients at initial presentation (2), as well as by susceptibility to infection. It is a medical emergency and an acute crisis state. Patients in a state of well-being between these episodes are referred to as “steady-state” SCD patients.The sequence of pathophysiological events that lead to the sickle cell VOC is not well understood. Several authors (8, 13, 27, 28) have outlined a sequence of steps occurring in the microcirculation that culminate in this painful sickle cell crisis. Polymerization of HbS, decreased blood red cell flexibility, microvascular occlusion, hypoxia of tissue involved with the occluded microvascular network, and tissue damage triggering painful stimuli have been mentioned (26), although the precise dynamics of these events and their interrelationships are poorly understood. Tissue ischemia due to vascular occlusion causing infarctive tissue damage, which in turn initiates secondary inflammatory responses, has also been mentioned (3, 4). Ischemic events produced by the occlusion of both large and small blood vessels are stressful and involve intricate interactions between red blood cells, the endothelium, and leukocytes (7). These interactions are known to be regulated by cytokines secreted by T cells as well as by adhesion molecules, and consequently, the immune response is implicated in the initiation and development of the sickle cell crisis. Indeed, studies now show that immune subsets are operative in sickle cell disease (9, 14, 16, 25), and the susceptibility of sickle cell disease patients in crisis to infections that specifically require the help of T cells to be cleared, such as Salmonella enterica serovar Typhimurium osteomyelitis (14), is suggestive.CD4⁺ T cells, subdivided based on their associated cytokines, play a crucial role in inflammatory responses and the elimination of infection. TH1 cells provide immunity against intracellular pathogens by secreting the cytokines interleukin-2 (IL-2), IL-12, and gamma interferon (IFN-γ), whereas commitment to the TH2 lineage programs the clearance of extracellular pathogens and the secretion of cytokines such as IL-10, IL-4, and IL-13. This balance of TH1/TH2 cytokine responses is believed to play an important role in coordinating an effective immune response, even under inflammatory conditions, although very limited data exist on their roles in sickle cell VOC.This study thus hypothesizes that the balance between TH1- and TH2-type cytokines might explain the differences in clinical outcomes in sickle cell disease. It was undertaken with patients with SCD in VOC in Zaria, Nigeria, a town in the zone of sickle cell endemicity of West Africa (17). The study analyzed numerical values for CD3⁺, CD4⁺, and CD8⁺ T cells and levels of selected serum cytokines in patients in VOC, and it compared these values with those obtained for steady-state SCD patients and unaffected hemoglobin AA homozygotes who served as normal healthy controls (NHC). This was done in an effort to understand if any imbalance in the immune response is important in the pathogenesis of sickle cell disease.