The impact of anaesthetics on immune function
Dr Koichi Yuki and his team at the Boston Children’s Hospital investigate the impact of anaesthesia on the immune system. Their research demonstrates that these medications, besides inducing anaesthesia, can also impact immune function. Not only that, but anaesthetic use could even worsen particular diseases during surgery, such as sepsis.
Surgical operations are usually performed under anaesthesia — an artificially-induced insensitivity to pain. There are different types of anaesthesia such as regional and general anaesthesia. Regional anaesthesia blocks nerve impulses from painful stimulation within a certain area while the patient retains consciousness and awareness. General anaesthesia, on the other hand, induces a state where the patient loses awareness and sensation of pain all over the body.
Anaesthesia in surgery
A variety of medications can be used to induce this general anaesthesia, called general anaesthetics, with most administered to patients intravenously or via inhalation. Medications that are given inhalationally are termed inhalational (volatile) anaesthetics and are a popular choice of drugs in surgery. Nonetheless, besides inducing anaesthesia, the effects of these drugs on the human body are often poorly understood. Interestingly, research is now showing evidence of interactions between anaesthetics and receptors/other proteins in the body’s immune cells.
Anaesthetics and the immune system
The immune system is a highly complex network of cells, organs and tissues that protects our body from foreign intruders like bacteria, viruses or fungi. Early research by Dr Yuki demonstrated that isoflurane and sevoflurane (commonly used volatile anaesthetics) can directly bind to and impair functions of a receptor (adhesion molecule) present on immune cells, called leukocyte function-associated antigen-1 (LFA-1). Isoflurane also impairs another adhesion molecule receptor macrophage-1 antigen (Mac-1).
Figure 1. The impact of isoflurane exposure on mortality in experimental polymicrobial abdominal sepsis. A–C, The outcomes of polymicrobial abdominal sepsis induced by CLP in WT mice without isoflurane exposure (n¼25), with isoflurane exposure for 2h (short exposure, n¼14) and 6h (long exposure, n¼20), LFA-1 KO mice withoutisoflurane exposure (n¼24), LFA-1 KO mice with isoflurane exposure for 6h (n¼16), Mac-1 KO mice without isoflurane exposure (n¼24) and Mac-1 KO mice with isoflurane exposure for 6h (n¼15) are shown. D, The outcomes of mice after CLP in WT mice with propofol exposure (n¼20) or without propofol exposure (n¼20) for 6h.Statistical significance was evaluated using Log-rank test. * and ** represent P < .05 and .01, respectively versus WT without isoflurane (or propfol). CLP, cecal ligation
LFA-1 is a surface protein present on all leukocytes. These are crucial cells in mediating various immune responses including leukocyte migration and immunological synapse formation. Mac-1 is present on a subclass of leukocyte, mostly innate immune cells. It mediates their recruitment and is involved in clearance of microbes and cell debris (through a process known as phagocytosis).
Dr Yuki and his team study the effect of these volatile anaesthetics on the immune system using two examples: destruction of tumour cells by immune cells, and sepsis which is a disease caused by dysregulated immune responses leading to a systemic inflammatory response upon infection.
Anaesthetics worsen sepsis
In addition to systemic inflammatory response, sepsis often causes life-threatening organ dysfunction. Severe sepsis and septic shock remains a health care burden and carries a significant high morbidity and mortality. It has a mortality rate of 20–30% and, in the US alone, around 750,000 cases of sepsis occur annually. More worrying though, these numbers are increasing worldwide. Sepsis is considered to be a time-sensitive disease and requires an urgent or emergent treatment upon diagnosis. Evaluation of the source of infection is one of the main treatment modalities. The removal of infected fluids or tissue is often performed under general anaesthesia using volatile anaesthetics.
Anaesthetics are typically administered to patients regardless of the type of disease being treated during surgical operations. However, as Dr Yuki’s research proves, certain anaesthetic drugs might not be suitable for anaesthetising certain patients.
In their recent research, Dr Koutsogiannaki (Instructor in Anaesthesia, Harvard Medical School), one of Dr Yuki’s lab members, demonstrated a negative effect of the volatile anaesthetic isoflurane on sepsis outcomes. LFA-1, the receptor shown to be affected by isoflurane in vitro, is important for recruitment of neutrophils (a subclass of leukocytes), the first-defence innate immune cells to inflamed tissue.
Figure 2. Cytotoxicity of K562 cells by NK92-MI cells was tested under different anesthetics at various concentrations. In addition, the effect of LFA-1 allosteric antagonist BIRT377 was tested. Cells were co-incubated for 4h. Data are shown as mean ± S.D. of 10 replicates for anesthetic experiments and 4 replicates for BIRT377 experiment. Statistical analyses were performed using one-way analysis of variance with Tukey post hoc analysis. *denotes p < 0.05 versus mock
Proinflammatory molecules activate LFA-1 present on neutrophils circulating in the blood stream. Activated LFA-1 then binds to intercellular adhesion molecules (ICAMs), which are present on the surface of cells on the interior of blood vessels (endothelial cells). Subsequently, neutrophils can traffic into the primary site of infection.
Dr Yuki and his team have demonstrated that, after administering isoflurane for six hours, significantly fewer neutrophils were recruited to septic tissue in their mouse model — in line with LFA-1 being hampered by the drug. However, intravenous anaesthetic propofol administration did not affect neutrophil recruitment to the primary site of infection. Similarly, Mac-1 was also hampered by isoflurane, with phagocytosis found to be less efficient and bacterial load higher in various tissues. The researchers also observed an increased mortality rate in septic mice exposed to isoflurane, but not to propofol. Interestingly though, a short isoflurane exposure did not worsen sepsis. Dr Yuki’s group postulated that this would be because bacteria grow exponentially and 6-hour exposure of isoflurane would be significantly long enough to attenuate neutrophil function and allow significant bacterial loads. This therefore indicates that the time of exposure to this anaesthetic probably plays an important role as well.
Anaesthetics prevent tumour cell death
Dr Yuki and his team found that different anaesthetics have different effects on another important set of immune cells called natural killer cells (NK cells). These cells are especially important due to their role in preventing tumour development, through their ability to directly kill tumour cells without being primed beforehand. This function is unlike other immune cells, which instead have to produce highly specific surface receptors in order to be able to recognise and kill target cells.
Dr Yuki and his team at Boston Children’s Hospital. Dr Yuki (on the left) and his team (from left to right) Wei Wang, Matt Chamberlain, Sophia Koutsogiannaki, Hui Zha, Kazumasa Tawaza and Erika Matsunami
Activated NK cells release toxic substances from cellular vesicles into their target cells, which, in turn, not only induces cell death, but also produces signals that stimulate a more specialised immune response. Interestingly, LFA-1 is one of the crucial activation receptors on NK cells and is also involved in binding to target tumour cells. Therefore, given that LFA-1 is affected by the volatile anaesthetics isoflurane and sevoflurane, Dr Yuki and his team have investigated the effect of these anaesthetics on NK cell function. By exposing co-cultured tumour and NK cells to isoflurane and sevoflurane, Dr Yuki and his team found that the tumour cell toxicity induced by NK-cells was reduced, as theorised. More specifically, the binding of NK-cells to their targets and the localisation of the cytotoxic vesicles within the NK cells was negatively affected.
Improving patient care
Anaesthetics are typically administered to patients regardless of the type of disease being treated during surgical operations. However, as Dr Yuki’s research proves, anaesthetics can affect immune functions and certain drugs might not be suitable for anaesthetising patients with certain disease states. His findings also provide incentives for patient-based studies, which will be necessary to finalise conclusions on optimal patient care.