*Purpose: Primary graft dysfunction (PGD) is a form of acute lung injury driven by ischemia-reperfusion mediated damage. An increase in plasma complement proteins – specifically C4a and C5a – is associated with PGD, and an increase in C3a and C5a levels is associated with mortality, independent of PGD. However, serum levels appear to be high at baseline post-lung transplantation (LTx); and hence, make it harder to distinguish patients at risk. Given that PGD manifests as an acute lung injury, we hypothesized that local complement activation would help identify those who go on to develop PGD post-LTx.

*Methods: Bronchoalveolar lavage (BAL) specimens were obtained from recipients at 2 and 24h post-LTx. PGD was scored based on the consensus definition at 24, 48 and 72h post-LTx by an investigator blinded to the results. Complement activation proteins were measured using ELISA. Specifically, levels of C4d, indicating classical/lectin pathway activation and soluble C5b-9 (sC5b-9), a marker of formation and cleavage of the membrane attack complex (MAC), were evaluated. Additionally, a novel ELISA developed in-house was used to quantify C3(H2O), the hydrolyzed form of C3 that is a trigger for alternative pathway activation and a source of intracellular C3 that modulates cytokine production by immune cells. Specific complement components were identified in “C4d high-sC5b-9 high” compared to “C4d low-sC5b-9 low” groups using multianalyte profiling. Results were expressed as median (interquartile range) and Spearman’s rho (r) was used for correlation.

*Results: 61 patients were recruited for the study. 32 had matched BAL specimens sequentially obtained at 2 and 24h post-LTx. We observed that the formation and cleavage of the MAC occurred as early as 2h post-LTx (as determined by BAL sC5b-9 levels), suggesting local complement activation in the lungs. Both C4d and C3(H2O) were also detected in the BAL at 2 and 24h post-LTx, and strongly correlated with sC5b-9 levels (r=0.59 for C4d and 0.54 for C3(H2O), n=72, p<0.0001). In those who did not develop PGD, sC5b-9 decreased to nearly undetectable levels at 24h post-LTx, compared to those who developed any PGD. This decrease from 2 to 24h in patients who did not develop PGD was also seen in BAL C4d and C3(H2O) levels. In comparison, the levels of C4d and C3(H2O) stayed elevated or increased from 2 to 24h in those who developed PGD. This association also held true for those developing severe PGD, and PGD occurring at 48 or 72h post-LTx, suggesting that this change in the levels of BAL complement proteins could identify those who would develop severe PGD even beyond 24h. Upon comparing individual complement protein levels between “C4d high-sC5b-9 high” compared to “C4d low-sC5b-9 low” groups using multianalyte profiling, the lectin pathway appeared to be activated in patients who developed PGD.

*Conclusions: Our initial analysis suggests that complement activation and MAC formation occurs in lungs post-LTx, can be detected as early as 2h post-LTx, and that the change in BAL complement protein levels from 2 to 24h helps identify recipients who develop PGD. Future work is necessary to incorporate these changes into a predictive model and validate it in an independent cohort for PGD risk.

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