In the present study, we found that 24 h after induction of AP, the pancreata of older animals have less edema, decreased inflammatory response and increased bacterial infiltration. This is associated with decreased expression of PAPs as measured by Western analysis and immunohistochemistry. While there were no differences between young and old animals in serum amylase and lipase activity, in the most-aged subgroup (c), there was an increase in CRPs.
These data suggest that the local response to AP is less severe in the older animals, and the systemic response may be worse. PAPs are endogenous protectors against pancreatic injury[14-18
], and it appears that the decrease in PAPs may not be related to the decreased inflammatory infiltrate. As will be described below, the effect may be a result of an altered balance of PAP 1 and 2. The decrease of PAPs may also result in increased bacterial infiltration into the pancreas, and ultimately a worsened outcome.
While the pancreas progressively atrophies with age, its reserve capacity results in minimal clinical changes. Studies comparing baseline and stimulated levels of pancreatic enzymes have shown no significant differences[24,25
]. However, the regenerative capacity of the pancreas is diminished[26,27
], as evidenced by decreased recovery from pancreatitis. At the molecular level, this has been attributed to decreased phosphorylation of AkT[27
Very little is known about the aged pancreas in AP. As we have observed in animals, in humans there seems to be a blunted local response and an exaggerated systemic one. Fan et al[4
] noted decreased local complications but increased systemic complications, leading to death. Kimura et al[28
] described elderly patients with high mortality from AP, but autopsy evidence revealed only mild interstitial pancreatitis, where the inflammation was centered in the ductal and interstitial tissues regions.
We postulated that as animals age, there would be loss of protective mechanisms from the pancreas which would lead to worsened fate after pancreatitis. We focused on the PAP family since we and others have shown that they are endogenous proteins which are protective in AP[14-16,29
]. We know that expression of other genes of the PAP family decrease with aging[20
], and we formulated experiments to determine whether PAPs decreased too.
We showed that in older rats subjected to NaT, there is blunted PAP1 and 2 protein response in both Western analysis and immunohistologic staining patterns. How does this correlate with the histologic and biochemical data above?
PAP1’s protective activity may be local or systemic. Locally, it has both mitogenic[30
] and anti-apoptotic[31
] effects, which affect regeneration after pancreatitis. PAP1 is a known anti-inflammatory agent[16
], and a potent modulator of lung injury in pancreatitis[29,32
]. Its loss with aging may worsen the systemic response after acute pancreatitis.
However, our results differ from others who observed increased local inflammatory infiltrates in PAP1 knockout mice[19
]. Our model differs not only in that we employed a different rodent species, but in the fact that other proteins, such as PAP2, 3 and other inflammatory modulators, are likely also depressed.
PAP2’s protective activity, on the other hand, is likely local. It can directly activate macrophages[17
], and macrophage activation has been shown to be protective in pancreatitis[33-35
]. PAP2 is a more potent immunomodulator than PAP1[36
]. In our present study we even noted that PAP2-positive pancreatic cells co-localize with leukocytes (Figure ); this was not observed with PAP1. We also noted a positive correlation of PAP2 with pancreatic MPO activity. Activation of macrophages can induce MPO activity[37,38
], and it is possible that a decrease of PAP2 will be associated with decreased macrophages or macrophage activity, which is detrimental for the pancreas and host. The decrease in PAP2 may really be the reason for the decreased leukocyte infiltration seen in older animals.
Finally, PAP proteins directly interact with bacteria. They can bind gram-negative[7
] and gram-positive bacteria[39
], and we have discovered a bacteriocidal action of PAP1 and 2 (unpublished results). Our data suggest there is more bacterial infiltration in the older pancreata, as evidenced by increased quantity of bacterial rRNA. The blunted PAP responses may result in more acute pancreatic infection.
Increased bacterial colonization could lead to an increased systemic response to the injury, which is worsened by decreased PAP1 level. It is possible that PAPs are endogenous bacteriostatic or bacteriocidal agents that protect against infection during AP. Whether the loss of PAP with aging leads to increased clinical pancreatic infection remains to be seen. To study this, longer term experiments along with qualitative analysis of bacterial species will be necessary.
In conclusion, this is the first study to show a physiologic difference in AP between young and old animals, and the first to show that endogenous pancreatic molecules are related to the diathesis of the disease. In older animals, there is decreased edema and leukocyte infiltration, but increased bacterial infiltration and even serum CRP level. All are associated with decreased endogenous PAPs in the pancreas. Further studies are planned on the direct effect of all three PAPs in modulating the severity of pancreatitis, by focusing on their local immunomodulatory effect and effect on bacterial infiltrates, as well as the systemic response of the host.