I used to use a L929 cell line for a TNF bioassay (not terribly sensitive).
Could any one comment on alternatives and sensitivity.

would this help you at all?

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=7930647&query_hl=1&itool=pubmed_docsum