Nerve growth factor : its contribution to neuroimmunomodulation

Abstract

Nerve growth factor (NGF) is the best characterised member of the neurotrophin family, a protein group with common structures and functions. Similar to other proteins of neuronal origin NGF, in addition to its well described action on neurons, is involved in the regulation of the immune system. NGF receptors are found on immune cells and autoimmune and inflammatory disorders show significant modifications in the basal concentrations of NGF.

The aim of this thesis was to investigate the role of NGF and neuropeptides in autoimmune and inflammatory disorders using in vivo and in vitro models.

In the kidneys of lupus prone mice (NZB/W female hybrids) increased concentrations of NGF and neuropeptides were found, whereas in the brain both NGF and neuropeptides were similarly decreased.

The spleen of NZB/W is characterised by extensive lymphoproliferation and a decrease in neuropeptides and increase of NGF levels during the disease progression.

The increase in NGF concentrations may be explained by the lack of NGF utilisation by neurons or by an increase in the local NGF production. Our results show that NZB/W splenocytes are immunopositive for NGF, and in their conditioned media of high concentrations of NGF were found.

Since in lupus, polyclonal B-lymphocyte activation and enhanced IgG production represent the main immune alteration, we elucidated the contribution of B-cells to the modified NGF synthesis found in lupus-prone mice. B-cells express NGF receptors and synthesise NGF and its production increased significantly after stimulation. Neutralisation of endogenous NGF using anti-NGF antibodies decreased the synthesis of bcl2 and induced DNA fragmentation, events that characterise apoptosis. The autocrine synthesis of NGF is essential for the survival of B-cells and it is possible that NGF may be one of the factors influencing the apoptosis and thereby the survival of autoreactive clones.

Neuropeptides can also influence lymphocyte development and functions. The synthesis of neuropeptides is a feature not only of neuronal cells but also of immune cells. In unstimulated B-cells there is a very low expression of calcitonin gene-related peptide (CGRP), but a stronger expression is induced after both in vivo and in vitro stimulation. Treatment with anti-NGF antibodies resulted in a marked reduction in CGRP expression in both resting and activated B cells. NGF appears to directly affect the synthesis of CGRP in B-cells in a similar way to that found in sensory neurons. By regulating CGRP synthesis in lymphocytes and neuronal cells, NGF may not only influence the intensity and duration of the immune response, but also exert a protective function.

Conclusions: The changes in NGF and neuropeptide basal concentrations observed during the progression of the disease in SLE mice seem to support the hypothesis that a modified communication between the nervous and immune systems contributes to the development of autoimmune diseases. The over-production of NGF found in SLE patients and murine models, by influencing lymphocyte survival, may interfere with the elimination of autoreactive clones.