RETNLB and pulmonary fibrosis: However, Fizz1 knockout mice challenged with the parasitic eggs of S. mansoni eggs displayed heightened pulmonary fibrosis associated with increases in CD4+ Th2 cell derived IL-4 and IL-13 [7,9] suggesting that Fizz1 is a negative regulator of pro-fibrotic Th2 cytokines induced by parasitic infections.