Priming can be defined as the induction of a state of hyper-reactivity to subsequent activating agents, and may be viewed on a continuum with activation and injury. Indeed many biologic agents that prime immune cells can also lead to activation at higher doses. Epidemiologic data first highlighted the requirement for immune priming in TRALI. A retrospective case-control study including ten TRALI patients demonstrated an underlying clinical morbidity such as sepsis, cancer, recent surgery, cytokine administration, or massive transfusion in 10/10 TRALI patients. By contrast, only 2/10 patients in the control group had identifiable morbidities (14
). A small case-control study by Sanchez et al
identified spine surgery as another potential recipient risk factor for TRALI (15
). Larger case-control series by Silliman et al
) and Gajic et al
), including 46 and 74 patients respectively, identified hematologic malignancy during the induction phase of chemotherapy, cardiac disease requiring surgery, sepsis, and chronic alcohol abuse as risk factors for TRALI. The results of a TRALI case-control study conducted by investigators at the University of California, San Francisco and the Mayo Clinic are expected soon and will have the potential to contribute a greater understanding of recipient and donor risk factors.
Animals models have validated hypotheses about priming that were generated based on the epidemiologic data. Silliman and colleagues have demonstrated induction of ALI ex vivo
in perfused and ventilated rat lungs using a variety of experimental agents, including purified lyso-PCs, the plasma fraction of older, stored platelets or packed red blood cells (PRBCs), and MHC Class I antibody (17
). Importantly, in each case, the observed effects required the pretreatment of the rats with intraperitoneal lipopolysaccharide (LPS). Further evidence for the role of priming comes from Sachs et al
). These investigators added fMLP (formyl-Met-Leu-Phe), a neutrophil priming and activating agent, during ex vivo
perfusion of rat lungs along with anti-HNA-2a antibody and neutrophils with high- or low-expressing cognate antigen. fMLP accelerated lung injury with the high-expressing cognate antigen neutrophils and also unmasked injury not previously observed with low-expressing cognate antigen cells.
In 2006, we reported the first mouse model of TRALI based on MHC Class I antibody challenge in BALB/c mice (21
). This model produced robust lung injury in the absence of an overt first event or priming step. However, we were subsequently surprised when we changed the housing conditions of our mice from non-barrier to barrier, specific-pathogen free rooms and the lung injury in our model significantly decreased () (22
). Lung injury was restored when the barrier mice were pre-treated with low-dose intraperitoneal (i.p.) or intratracheal (i.t.) LPS prior to antibody challenge. Interestingly, the barrier mice had significantly lower neutrophil counts in the peripheral blood compared with the non-barrier animals () (22
). Priming with LPS increased circulating neutrophil counts and increased sequestration of neutrophils in lung microcirculation. From these experiments, we concluded that environmental conditions could significantly influence TRALI susceptibility through modulation of the neutrophil response in the peripheral blood and the lung microcirculation.
Effect of Immune Priming in anti-MHC Class I-mediated TRALI
models have also shed light on the mechanism of priming and its relationship to lung injury. In response to inflammatory cytokines, neutrophils and the pulmonary endothelium undergo adaptive changes that result in neutrophil sequestration in the lung microvasculature. Under normal physiologic conditions, neutrophils deform and elongate to fit through the many, narrow lung capillary segments (23
). However, after inflammatory challenge with fMLP (23
), IL-8 (24
), activated plasma (25
) or LPS (21
), neutrophils sequester in the lungs through both mechanical and adhesive mechanisms. Re-organization of cellular actin results in stiffening of the cell membrane and the neutrophils become lodged in the pulmonary microvasculature (23
). However, cytoskeletal reorganization does not result in permanent sequestration without subsequent changes in endothelial and neutrophil adhesion molecules (24
Augmented neutrophil and endothelial expression of adhesion molecules may be required for prolonged neutrophil recruitment to the lungs. Blocking neutrophil L-selectin or CD11/CD18 (Mac-1) does not alter the rate of sequestration, but does alter retention time (23
). Endothelial cells increase expression of Inter-Cellular Adhesion Molecule 1 (ICAM-1) in response to LPS treatment or thromboxane (27
), and adding monoclonal antibody against CD18 or ICAM-1 to human microvascular endothelial cell (HMVEC) and neutrophil co-cultures blocks neutrophil adhesion and lyso-PC stimulated damaged of HMVECs (27
). E-selectin is not constitutively expressed on the pulmonary endothelium, but it can be induced on the pulmonary endothelium, along with E-selectin ligand-1 (ESL-1) on neutrophils, by a number of inflammatory stimuli including TNF-α, IL-1, G-CSF, and IL-17 (29
), and may mediate neutrophil secondary capture of additional cellular elements to sites of inflammation (32
In addition to sequestering neutrophils in the lung microvasculature, priming agents may also augment subsequent neutrophil respiratory burst (27
). Wyman et al
showed that pretreatment with LPS potentiated subsequent lyso-PC- or fMLP-induced superoxide and elastase production and augmented neutrophil mediated HMVEC injury in vitro
). This same group also demonstrated that pretreatment of HMVECs with LPS and subsequent co-culture with anti-HNA-3a antibody and cognate antigen positive neutrophils potentiates HMVEC cellular injury (27
). Thus, pretreatment with LPS may augment neutrophil-mediated killing through both neutrophil-dependent and endothelium-dependent mechanisms.
Immune priming may affect cellular recruitment through stiffening of neutrophils and increased expression of surface adhesion molecules. It may also affect the magnitude of injury through modulation of neutrophil effector functions, increased expression of cognate antigens, or recruitment of additional cellular elements necessary for injury amplification. Of note, in our MHC Class I antibody model of TRALI, increased expression of cognate antigen with LPS priming was not observed in lung endothelial cells or neutrophils (22
). However, increased cognate antigen expression is still a plausible mechanism of priming in TRALI cases with MHC Class II or HNA antibody. Additional experiments should define the full range of factors that are capable of priming animals for TRALI, including modeling of trauma or surgical intervention and further exploration of the role of bacterial infection on leukocyte-endothelial interactions.