Role of leukocyte integrins in experimental models of Alzheimer’s disease

Alzheimer’s disease (AD) is characterized by severe, progressive decline of cognition due to neuronal loss in brain regions involved in learning and memory. Two main pathophysiological hallmarks of AD are well characterized: amyloid beta (Aβ) plaques and neurofibrillary tangles (NFTs) of hyperphosphorylated tau protein. Significant evidence obtained over the last decade has shown that neuroinflammation is also associated with AD pathology. Furthermore, vascular leakage and endothelial activation were reported in AD brains, suggesting a role for vascular inflammation and leukocyte trafficking in the pathogenesis of AD. However, the inflammation mechanisms involved in AD pathogenesis remain largely unknown and a better understanding of the role of inflammation in AD may help to develop new therapeutic approaches to slow the progression of this disorder. Bloodderived leukocyte subpopulations, including lymphocytes, monocytes, and neutrophils, have been identified in the brains of patients with AD and in corresponding animal models, but their role in disease pathogenesis is unclear. We have recently reported an active inflammatory process taking place during AD, which includes up-regulation of adhesion molecules on cerebrovascular endothelium and leukocyte trans-endothelial migration into the brain of AD-like disease mice. Notably, neutrophil depletion during the early phase of disease led to an amelioration of cognitive deficits and neuropathological condition in mouse models of AD, suggesting their contribution to the pathology. Indeed, the inhibition of neutrophil function strongly reduced microglial activation and Aβ deposition, suggesting that neutrophils play a key role in disease progression. The GOAL of the present study was to investigate the role of LFA-1 and VLA-4 integrins in the pathogenesis of AD-like disease. LFA-1, the predominant β2-integrin expressed on leukocytes, is known to play a key role in leukocyte adhesion on inflamed endothelium. Instead VLA-4, the predominant α4-integrin expressed on the surface of lymphocytes, is expressed only by approximately 3% of circulating neutrophils, but several studies demonstrated it represents an alternative pathway for neutrophil migration to inflammatory sites. Interestingly, in addition to neutrophil infiltration we found T cells infiltrating the brain in mouse models of AD at different time points of disease. Therefore, different leukocyte subpopulations migrate into the brain of 3xTg-AD mice, suggesting that neutrophils and T cells may play a role in disease evolution. We performed our experiments in 3xTg-AD mice, which reproduces AD-like cerebral amyloidosis and tangle pathology, and closely resemble the cognitive and behavioral alterations reported in human disease. We first evaluated by two-photon microscopy the effect of monoclonal antibodies known to block the integrin LFA-1 on neutrophil thus affecting adhesion and extravasation in the central nervous system (CNS). Our results showed that LFA-1 integrin blockade prevents neutrophil adhesion, extravasation, and inhibits intraparenchymal motility in the brain of 3xTg-AD mice. In addition, we demonstrate that both oligomeric and fibrillary Aβ are able - 4 - to trigger rapid LFA-1- dependent neutrophil adhesion to its counterligand ICAM-1, with oligomeric Aβ preparation being more effective. Subsequently, we assessed the blockade of neutrophil trafficking by an anti-LFA–1 integrin antibody treatment in 3xTg-AD mice at early stages of the disease confirming that the integrin LFA-1 is fundamental for brain infiltration of neutrophils in AD-like mice. We next studied the effects of LFA-1 ablation on 3xTg-ADxItgal-/- mice lacking LFA-1 integrin. We found that they show improved memory in cognitive tests compared to wild-type animals. Our results demonstrated a reduced of cognitive dysfunctions in 3xTg-ADxItgal-/- compared to age-matched 3xTg-AD mice. These findings were supported by neuropathological data showing a lower density and activation state of microglia in the CA1 and DG and a reduction of Ab deposition and tau hyperphosphorylation in 3xTg-ADxItgal-/- compared to 3xTg-AD aged-matched controls. Taken together, these results suggest that the inhibition of neutrophil trafficking through the blockade of LFA-1 integrin may represent a new therapeutic strategy for AD. In addition, our results showed that treated 3xTg-AD mice with an anti-a4 integrin antibody improved memory function compared to control treated mice in behavioral tests. Notably, restoration of cognitve function in mice that received anti-a4 treatment during early stages of disease was also maintained at late time points in aged animals, suggesting that therapeutic blockade of leukocyte adhesion during the early stages of disease has a longterm beneficial effect on cognition in older mice. In support of the results obtained in cognitive tests, neuropathological studies showed a reduction of amyloid beta deposition, tau hyperphosphorylated and microglial activation. Therefore, VLA-4 integrin may also play a key role in the induction of cognitive deficit and progression of AD-like disease. In summary, the results obtained in the present study show that LFA-1 and VLA-4 integrins contribute to disease pathogenesis and may represent novel therapeutic targets in AD.