LYMPHOID-TISSUES INDUCE NGF-DEPENDENT AND NGF-INDEPENDENT NEURITE OUTGROWTH FROM RAT SUPERIOR CERVICAL-GANGLIA EXPLANTS IN CULTURE

Induction of neurite outgrowth from superior cervical ganglia (SCG) by rat lymphoid tissues was studied using a tissue culture model. Neonat al rat SCG were cultured with 6-12-week-old rat thymus, spleen, or mes enteric lymph node (MLN) explants in a Matrigel layer, in defined cult ure medium without exogenous nerve growth factor (NGF). SCG were also co-cultured with neonatal rat heart (as positive control) or spinal co rd (SC; as negative control). To determine whether inflammation affect s the ability of lymphoid tissues to induce neurite outgrowth, we also examined MLN at various times after infecting rats with Nippostrongyl us brasiliensis (Nb-MLN). In one series of experiments, a single lymph oid tissue explant was surrounded by four SCG at a distance of 1 mm. T he extent of neurite outgrowth was determined by counting the number o f neurites 0.5 mm away from each ganglion at several time points. Adul t thymus and, to a lesser extent, spleen had strong stimulatory effect s on neurite outgrowth from SCG after 12 hr or more in culture. For th ymus tissue, this was similar to the positive control heart explants. MLN from normal rats had minimal effect on neurite outgrowth; however, Nb-MLN showed a time-dependent enhancement of the neurite outgrowth, maximal at 3 weeks after infection. The relative efficacy of neurite o utgrowth induction (heart greater than or equal to thymus greater than or equal to Nb-MLN spleen greater than or equal to MLN greater than o r equal to SC) was confirmed in a second series of experiments where o ne SCG was surrounded by three different tissue explants, We then exam ined the role of 2.5S NGF, a well-known trophic factor for sympathetic nerves, in the lymphoid tissue-induced neurite outgrowth. Anti-NGF tr eatment of co-cultures of SCG and heart almost completely blocked the neurite outgrowth. Anti-NGF also significantly inhibited thymus- and s pleen-induced neurite outgrowth, but not as effectively as heart-induc ed neuritogenesis (93, 80, and 77% inhibition at 24 hr; 86, 70, and 68 % inhibition at 48 hr for heart, thymus, and spleen, respectively). On the other hand, anti-NGF inhibited only 8% of neurite outgrowth induc ed by 3-week post-infection Nb-MLN at 24 hr, and 41% at 48 hr. These d ata show that several adult rat lymphoid tissues exert neurotrophic/tr opic effects. The predominant growth factor in thymus and spleen is NG F, while Nb-MLN produces factor(s) which is (are) immunologically dist inguishable from NGF. These neurotrophic/tropic factors are produced d uring the reactive lymphoid hyperplasia that forms part of the inflamm atory response against the nematode, N. brasiliensis. This suggests th e possibility that cytokines produced by lymphocytes or other inflamma tory cells may stimulate sympathetic neurite outgrowth in vivo. (C) 19 94 Wiley-Liss, Inc.