Trying to hint an index usage fails: "Failed to fulfil the hints of the query. Could not solve these hints: `USING INDEX"

Assuming you are hinting a text index, it will handle text values only, it looks like the condition for the index to be used (whether automatically or forced) is for the value used for the lookup to either be cast as a text type, or to use a string literal within the query.
Adding a toString() around the value to use forces the casting and allows expected index lookup to work.
 

MATCH (cs:cell:CD_Ileum:Normal {cell_id: toString(line.source)})
USING INDEX cs:cell(cell_id)
MATCH (ct:cell:CD_Ileum:Normal {cell_id: toString(line.target)})
USING INDEX ct:cell(cell_id)

 
This will work with or without the index hint.