Direct interactions between immunocytes and neurons after axotomy in Aplysia

Axon growth during development and after injury has processes in common, bu t also differs in that regeneration requires the participation of cells of the immune system. To investigate how neuron-immunocyte interactions might influence regeneration, we developed an in vitro model whereby neurons and hemocytes from Aplysia californica were cocultured. The hemocytes, which be have like vertebrate macrophages, migrated randomly throughout the dish. Wh en a neuron was encountered, some hemocytes exhibited an avoidance response , whereas others formed stable contacts. Hemocytes did not distinguish betw een neurons from different animals. Stable contacts occurred on neurites an d growth cones, but not the cell soma, and were benign in that the hemocyte s did not impede neurite growth. When hemocytes attached to the cell body, it presaged the destruction of the neuron. Destruction was a dynamic proces s that was initiated when groups of one to three hemocytes adhered to vario us regions of the cell soma. Each group was then joined by other hemocytes. They did not contact the neuron, but interconnected the initial groups, fo rming a network around the neuron. The network then contracted to dismember the cell. Once a neuron was destroyed, hemocytes removed the debris by pha gocytosis. Both damaged neurons and those without apparent damage were targ ets for destruction. Severing neurites with a needle resulted in the destru ction of only one of six cells. Our studies suggest that hemocytes, and by extrapolation, vertebrate macrophages, exhibit highly complex interactions with neurons that can exert a variety of influences on the course Of nerve regeneration, (C) 2001 John Wiley & Sons, Inc*.