TGF-β1 in SP-A preparations influence immune suppressive properties of SP-A on human CD4⁺ T lymphocytes

Surfactant protein A (SP-A) and transforming growth factor- β₁ (TGF-β₁) have been shown to modulate the functions of different immune cells and specifically to inhibit T lymphocyte proliferation. The aim of the present study was to elucidate whether the Smad signaling pathway, which is activated by TGF-β1, also plays a role in SP-A-mediated inhibition of CD4⁺ T lymphocyte activation. Recombinant human SP-A1 expressed in Chinese hamster ovary cells [rSP-A1m (mammalian)], but not recombinant Baculovirus-derived rSP-A1^hyp (hydroxyproline- deficient), suppressed T lymphocyte proliferation and IL-2 mRNA expression. To test whether SP-A induced Smad signaling, a Smad3/4-specific reporter gene was transfected in primary human CD4⁺ T lymphocytes. Only rSP-A1m, but not rSP-A1^hyp, induced Smad-specific reporter genes, Smad2 phosphorylation, and Smad7 mRNA expression. The effect of rSP-A1m was mediated through the TGF-βRII and could be antagonized by anti-TGF-β1 neutralizing antibodies and sTGF-βRII. Western blot and ELISA analysis revealed that rSP-A1m, but not rSP-A1^hyp, contained TGF-β1. TGF-β1 was responsible for the differences in inhibition of CD4⁺ T lymphocyte proliferation and activation of the Smad signaling pathway between rSP-A1m and rSP-A1^hyp. After acidification, native SP-A, obtained from patients with alveolar proteinosis, also induced Smad signaling in human CD4⁺ T lymphocytes leading to an increased inhibition of T lymphocyte proliferation, thus indicating the presence of inactive, latent TGF-β1 in native SP-A samples. Association between SP-A and latent TGF-β1 provides a possible novel mechanism to regulate TGF-β1-mediated inflammation and fibrosis reactions in the lung but also leads to possible misinterpretation of immune-modulator functions of SP-A. Monitoring of SP-A preparations for possible TGF-β1 is essential.