Steroid Resistant CD8+CD28null NKT-Like Pro-inflammatory Cytotoxic Cells in Chronic Obstructive Pulmonary Disease

Abstract

Corticosteroid resistance is a major barrier to effective treatment in chronic obstructive pulmonary disease (COPD), and failure to suppress systemic inflammation in these patients may result in increased comorbidity. Although much of the research to date has focused on the role of macrophages and neutrophils involved in inflammation in the airways in COPD, recent evidence suggests that CD8⁺ T cells may be central regulators of the inflammatory network in this disease. CD8⁺ cytotoxic pro-inflammatory T cells have been shown to be increased in the peripheral blood and airways in patients with COPD, whereas smokers that have not progressed to COPD only show an increase in the lungs. Although the mechanisms underlying steroid resistance in these lymphocytes is largely unknown, new research has identified a role for cytotoxic pro-inflammatory CD8⁺ T-cells and CD8⁺ natural killer T-like (NKT-like) cells. Increased numbers of these cells and their significant loss of the co-stimulatory molecule CD28 have been shown in COPD, consistent with findings in the elderly and in clinical conditions involving chronic activation of the immune system. In COPD, these senescent cells expressed increased levels of the cytotoxic mediators, perforin and granzyme b, and the pro-inflammatory cytokines, IFNγ and TNFα. They also demonstrated increased cytotoxicity toward lung epithelial cells and importantly were resistant to immunosuppression by corticosteroids compared with their CD28⁺ counterparts. Further research has shown these cells evade the immunosuppressive effects of steroids via multiple mechanisms. This mini review will focus on cytotoxic pro-inflammatory CD8⁺CD28^null NKT-like cells involved in COPD and novel approaches to reverse steroid resistance in these cells.

Keywords: CD28; CD8+ NKT-like cell; HDAC2; Hsp90; IFNγ and TNFα; Pgp; chronic obstructive pulmonary disease; steroid resistance.

Figure 1

Flow cytometric gating technique used to identify CD8⁺CD28^null natural killer T-like (NKT-like) cells (and CD8⁺CD28⁺ NKT-like cells) from the peripheral blood of patients with chronic obstructive pulmonary disease. (A) Identification of lymphocytes as CD45⁺ low side scatter (SSC) events. (B) Removal of red blood cell contaminations removed by lymphocyte gating using forward scatter vs. SSC characteristics. (C) Identification of NKT-like cells as CD3⁺CD56⁺ events. (D) Identification of CD28^null and CD28⁺ NKT-like cells using CD8 vs. CD28 staining. (E) Expression of IFNγ and histone deacetylase (HDAC)2 in CD8⁺CD28^null and CD8⁺CD28⁺ cells. (F) Expression of P-glycoprotein-1 (Pgp1), glucocorticoid receptor (GCR), and heat shock protein (Hsp)90 expression in CD8⁺CD28^null and CD8⁺CD28⁺ cells. Note: the majority of NKT-like cells are CD8⁺ and CD28^null. These cells express reduced HDAC2, GCR, and Hsp90 but increased IFNγ compared with CD8⁺CD28⁺ NKT-like cells (Pgp1 unchanged).

Figure 2

Schematic diagram summarizing reported findings in peripheral blood CD8⁺CD28^null natural killer T-like (NKT-like) cells in chronic obstructive pulmonary disease (COPD). Glucocorticoids enter cells by overcoming membrane drug efflux pump P-glycoprotein-1 (Pgp1) and binding to the glucocorticoid receptor (GCR) in the cytoplasm. GCR must be bound to the molecular chaperones heat shock protein (Hsp)70 and Hsp90 to acquire a high-affinity steroid binding conformation, and traffic to the nucleus where engagement of histone deacetylases (HDACs), particularly HDAC2, results in reduction of pro-inflammatory gene activation. In COPD compared with age-matched healthy control subjects: (A) Pgp1⁺ NKT-like cells are increased in COPD, reducing intracellular levels of GC. Expression of GCR (C), Hsp90 (F), and HDAC2 (D) are decreased in CD8⁺CD28^null NKT-like cells (no change in Hsp70) (G) reducing steroid effectiveness. (I) The percentage of steroid resistant CD8⁺CD28^nullCD137⁺ NKT-like cells is increased. Possible therapeutic targeting to overcome steroid resistance CD8⁺CD28^null NKT-like cells in COPD: (B) Pgp1 is synergistically decreased in the presence of 2.5 ng/ml cyclosporine A (CsA) and 1µM prednisolone (pred). (H) Hsp90 expression is increased in the presence of 2.5 ng/ml CsA and 1µM pred. (E) HDAC2 expression is increased in the presence of 5 mg/ml theophylline, 2.5 ng/ml CsA, and 1µM pred. (J) Blocking CD137 expression with anti-CD137 antibody. (K) This targeting results in decreased IFNγ and TNFα pro-inflammatory cytokine expression.