PON-1 has an established role in modulating oxidative stress, which is a major promoter and mediator of the systemic inflammatory response occurring in sepsis (17). The activity of PON-1 during septic inflammation has been evaluated in other species (18,20,25,29,30), but not in horses yet. We investigated plasma PON-1 activity following intravenous injection of 30 ng/kg of LPS (E. coli O55:B5) in healthy horses included in a previous study (31). The authors of the previous study demonstrated significant clinical changes caused by endotoxemia within the first 12 hours post-infusion, significant changes in WBC counts 6 hours and marked neutrophilia at 24 hours after LPS infusion. Then, all these changes normalized in all horses after 48 hours. Plasma iron concentration significantly decreased to lowest values at 12 hours post-infusion and raised again starting from 24 hours, returning to baseline values at 36 hours. As expected, we observed a decrease in PON-1 activity. Differently from clinical signs and changes in WBC and plasma iron reported in the previous study (31), PON-1 activity recorded in the present study did not show significant changes in the first 12 hours. On the contrary, a significant decrease in PON-1 activity was recorded starting from 48 hours post-infusion, when all the other clinical and analytical parameters had returned to normal reference intervals. The simultaneous decrease in PON-1 activity and recovery from clinical signs is in contrast with those studies that in humans reported PON-1 increase when patients with naturally occurred sepsis clinically improved (18,19,22). The same studies however (19,22), reported a sustained decrease in PON-1 activity for several days, which we also observed. Our study is based on an experimental model of endotoxemia, which does not completely mimic the conditions and pathologic pathways of naturally occurring endotoxemia. In the previous study (31) the evaluation of short-term effects of LPS infusion (i.e. 0-6 hours after LPS administration) was based on clinical changes and not on a complete SIRS score, that also includes the evaluation of WBC count. However, all the horses had at least 2 alterations of clinical parameters consistent with SIRS status evaluation proposed by Roy et al. (32) and therefore it can be assumed that LPS administration induced a systemic inflammatory reaction. Our results agree with other papers assessing PON-1 activity in experimentally induced endotoxemic animals. In dogs, Tvarijonaviciute et al. (20) reported normalisation of clinical signs in 48 hours after LPS administration (although clinical signs occurred earlier compared with horses included in our study), and decreased PON-1 activity at 30 hours, which progressed and became significant at 48 hours compared to 24 hours; a similar trend to what we observed. These authors did not measure PON-1 activity over 48 hours post LPS administration, so a comparison of long-term normalisation of PON-1 activity cannot be made. Feingold et al. (27) found in Syrian hamsters a progressive decrease in PON-1 activity, already significant 8 hours after LPS administration, which even in this case persisted at least up to the last sampling at 48 hours. These and our results seem to state that experimentally induced endotoxemia induces changes in PON-1 activity that does not have a rapid resolution, contrarily to clinical signs, that normalize earlier. This latter discrepancy may explain the lack of significant correlations between PON-1 activity and the clinical score. Our data seem to state that endotoxemia triggers changes in PON-1 activity later and for a longer period, compared with leukocyte count and plasma iron concentration. Again, this delayed response may explain the lack of correlations between PON-1 activity and many laboratory changes, except for fibrinogen that, being a positive acute phase protein, negatively correlated, as expected, with PON-1 activity and for the PCV that increased soon after LPS infusion likely due to dehydration and then decreased to normal values, thus paralleling the late decrease of PON-1. This late PON-1 response could be interpreted as a delayed occurrence of inflammatory oxidations and oxidative stress, possibly due to a different PON-1 metabolism during inflammation in horses compared to other species, as suggested by Ruggerone et al. (15). Otherwise, it may also be possible that anti-inflammatory oxidations occurred in the first 12 hours post-LPS infusion, determining PON-1 activity fluctuations not wide enough to produce significant changes, possibly because of a less severe oxidative stress in horses compared to other species (Ruggerone et al., 2020). A reduced hepatic synthesis, due to cytokine down regulation or to liver function impairment after LPS administration, could have determined the subsequent noticeable decrease of PON-1 activity from the third day up to the seventh. Feingold et al. (27) simultaneously evaluated also liver PON-1 mRNA expression and observed an earlier and greater decrease in mRNA level, starting from 4 hours post-LPS administration and reaching lowest values at 16 hours, followed by a progressive increase which did not reach baseline values at 48 hours. This led to the hypothesis that acute changes in PON-1 production mildly affect serum levels, possibly because of a relatively long half-life of the enzyme (27). Liver expression of PON-1 mRNA was not evaluated in our study, but a similar mechanism would explain the delayed and sustained decrease in PON-1 activity that followed clinical recovery and normalisation of other laboratory parameters. Investigation of PON1 liver expression in the future may clarify the causes of delayed and prolonged decrease of PON-1. Furthermore, PON-1 activity is dependent on the lipid and protein composition of HDL (33), and correlation with lipid profile and other markers of inflammation such as serum amyloid A, may help acquiring knowledge about PON-1 metabolism in this species.
Biomarkers of sepsis so far evaluated in horses include interleukin-1β (IL-1β), interleukin-10 (IL-10), interleukin-6 (IL-6), serum amyloid A (SAA), soluble CD14 (sCD14) and procalcitonin (34,35). Measurement of sCD14 provided contrasting results: no significant differences in concentration were recorded among time in experimental settings (34), while sCD14 was higher in in spontaneous clinically endotoxemic than nonendotoxemic horses, showing however no correlation with LPS concentration (36). In many studies involving different types of experimentally induced endotoxemia in horses, cytokine expression was rapidly upregulated. Following LPS infusion, IL-1β and IL-6 gene expression peaked at 60 and 90 minutes respectively and decreased to still significantly higher levels than baseline 3 hours post infusion (37). In a similar way, serum TNF-α increases within 30 minutes, peaks at 60 minutes, and decreases 2-3 hours from endotoxin administration (37,38). Even if studies about SAA in experimentally induced endotoxemic horses are not available, blood SAA concentration is reported to increase up to 1000 times 6 hours after inflammatory stimuli and decrease within 12 hours after stimulus removal (39–41). It is not known if infection causes an earlier or persistent increase in SAA concentration compared with non-infectious inflammation, but there is evidence that infections are particularly effective in producing strong SAA responses (42), especially in neonatal sepsis (43). Since SAA is produced by the liver in response to cytokines stimulation, it is reasonable to think its increase in circulation be delayed. A decrease in PON-1 activity would be expected together with the early increase in SAA levels, since during the acute phase response the latter displaces PON-1 in HDL (44) and these two acute phase proteins were reported to be inversely correlated (45). However, cytokine-dependent hepatic regulation could have major impact on PON-1 activity rather than displacement by SAA (46), and this would explain the delayed decrease of PON-1 activity, as reported above. Comparing changes in PON-1 activity detected in this study and changes in other markers investigated in experimentally-induced endotoxemic horses, it seems that other markers normalise before PON-1 decreases. In fact, CCL-2 and IL-10 concentrations increased, peaked and decreased within 24 hours post-infusion. Procalcitonin as well showed a similar trend, but its concentration, even if decreased, was still statistically higher than baseline 24 hours post-infusion (34). However, endotoxin is a single factor implicated in the development of sepsis, and it is well known that differences exist between experimentally-induced and clinical endotoxemia and sepsis (47). In humans, PON-1 activity and CCL-2 concentration have an inverse correlation during natural infection (48) and CCL-2 decreases together with the increase of PON-1 and the resolution of naturally occurring septic processes after several days of hospitalisation (19). Investigation and comparison of changes in these markers during natural infection in horses would clarify their behaviour, relationship and usefulness in this species. This study has some limitations. Firstly, the long storage of samples that may have in part affected the magnitude of PON-1 activity. A previous study demonstrated that 6 months storage at -20°C can artifactually slightly increase PON-1 activity in dogs (49). However, no information on storage stability at -80°C, or storage studies on equine PON-1 are available. However, as a support of the hypothesis of a storage effect, PON-1 activity was lower than the reference intervals (28) in most of the horses at T0 despite the lack of any clinical or laboratory abnormality associated with inflammation. Nevertheless, this effect, if any, was present for all the samples included in this study and despite possible storage artifacts, fluctuations of PON-1 activity consistent with endotoxin-induced inflammation were present. A second limitation is the low caseload, coupled with the lack of a control group inoculated with a sham solution: this approach, that however, is in line with other studies based on a complex study design and is supported also by ethical reasons. However, also in this case, such a low but well standardized caseload, on which post-inoculation results were compared with baseline values recorded before LPS administration, allowed to have a better overview of the possible effect of LPS infusion on PON-1 activity compared with in field studies, on which several variables cannot allow to evaluate the direct effect of sepsis on clinical and laboratory parameters. Undoubtedly, following an inflammatory stimulus of septic type, such as the administration of endotoxins, PON-1 activity decreases, as reported for other species. According to our results, plasma iron concentration appears to be an earlier marker of sepsis onset and positive resolution compared to PON-1 activity. However, all horses showed a normalisation of PON-1 activity to baseline values 10 days after LPS administration. As claimed by Oliveira-Filho et al. (31), the inflammatory model adopted by the authors is safe and efficient and caused an acute systemic inflammation without prolonged inflammatory effects. As noted above, in naturally occurring sepsis, clinical and pathological conditions are not as well standardised. Therefore, in natural situations, where the extent and severity of inflammation (17) are very variable, PON-1 might prove to be useful as a diagnostic or prognostic marker of sepsis. In this case, measurement of PON-1 activity could be useful where limitations of iron measurement in detecting systemic inflammation exist, for example during corticosteroid administration, iron supplementation, haemolysis and age-relate variability (50).