Haploinsufficiency of NADPH Oxidase Subunit Neutrophil Cytosolic Factor 2 Is Sufficient to Accelerate Full-Blown Lupus in NZM 2328 Mice.

OBJECTIVE: We have previously established that the gene for neutrophil cytosolic factor 2 (NCF-2) predisposes to lupus, and we have identified lupus patients with point mutations that are predicted to cause reduced NADPH oxidase activity. We undertook this study to investigate the relationship between reduced leukocyte NADPH oxidase activity and immune dysregulation associated with systemic lupus erythematosus (SLE). METHODS: We generated NCF-2-null mice, in which NADPH oxidase activity is absent, on the nonautoimmune C57BL/6 (B6) mouse background and on the NZM 2328 mouse background, a polygenic model in which mice spontaneously develop lupus. Clinical disease, serology, and immunopathology were evaluated. RESULTS: NCF-2-null mice on the B6 background were susceptible to Aspergillus fumigatus pneumonia characteristic of chronic granulomatous disease, but did not develop systemic lupus disease. In contrast, NCF-2-null and even NCF-2-haploinsufficient mice on the NZM 2328 background developed accelerated full-blown lupus with significantly accelerated lupus kidney disease. This was characterized by more rapid development of hyperactive B cell and T cell immune compartments, increased expression of type I interferon-responsive genes, and generation of neutrophil extracellular traps, which were observed even in the absence of NADPH oxidase activity. CONCLUSION: Just as patients with chronic granulomatous disease who lack NADPH oxidase rarely develop SLE, NCF-2-null mice on a nonautoimmune background were susceptible to a chronic granulomatous disease-like opportunistic infection but did not develop lupus. In contrast, on a lupus-prone background, even haploinsufficiency of NCF-2 accelerated the development of full-blown lupus disease. This establishes an interaction between reduced oxidase activity and other lupus-predisposing genes, paralleling human SLE-associated variants predicted to have only reduced NADPH oxidase activity.

Once phosphorylated, tyrosines in carboxyl terminus of protein-tyrosine kinase Syk interact with signaling proteins, including TULA-2, a negative regulator of mast cell degranulation.

Activation of the high affinity IgE-binding receptor (FcεRI) results in the tyrosine phosphorylation of two conserved tyrosines located close to the COOH terminus of the protein-tyrosine kinase Syk. Synthetic peptides representing the last 10 amino acids of the tail of Syk with these two tyrosines either nonphosphorylated or phosphorylated were used to precipitate proteins from mast cell lysates. Proteins specifically precipitated by the phosphorylated peptide were identified by mass spectrometry. These included the adaptor proteins SLP-76, Nck-1, Grb2, and Grb2-related adaptor downstream of Shc (GADS) and the protein phosphatases SHIP-1 and TULA-2 (also known as UBASH3B or STS-1). The presence of these in the precipitates was further confirmed by immunoblotting. Using the peptides as probes in far Western blots showed direct binding of the phosphorylated peptide to Nck-1 and SHIP-1. Immunoprecipitations suggested that there were complexes of these proteins associated with Syk especially after receptor activation; in these complexes are Nck, SHIP-1, SLP-76, Grb2, and TULA-2 (UBASH3B or STS-1). The decreased expression of TULA-2 by treatment of mast cells with siRNA increased the FcεRI-induced tyrosine phosphorylation of the activation loop tyrosines of Syk and the phosphorylation of phospholipase C-γ2. There was parallel enhancement of the receptor-induced degranulation and activation of nuclear factor for T cells or nuclear factor κB, indicating that TULA-2, like SHIP-1, functions as a negative regulator of FcεRI signaling in mast cells. Therefore, once phosphorylated, the terminal tyrosines of Syk bind complexes of proteins that are positive and negative regulators of signaling in mast cells.

The limited contribution of Fyn and Gab2 to the high affinity IgE receptor signaling in mast cells.

Several studies with mast cells from knock-out mice have suggested that the tyrosine kinase Fyn and its downstream substrate Gab2 may play a role in high affinity IgE receptor (FcepsilonRI)-mediated mast cell activation. To better understand the role of these two molecules and of Syk, we transiently transfected mast cells with small interference RNA (siRNA) targeted to Fyn, Gab2, or Syk to specifically decrease their expression. The siRNA suppression of Gab2 but not Fyn reduced activation of the phosphoinositide-3-kinase (PI3K) pathway as demonstrated by the change in phosphorylation of Akt; this indicates that Gab2 but not Fyn regulates this pathway. The decreased expression of Gab2 and Fyn had minor effects on degranulation. There were also some minor changes in activation of the NFAT or NFkappaB transcription factors in cells with reduced expression of Fyn or Gab2. Decreased Gab2 but not Fyn reduced the FcepsilonRI-induced activation of the Erk, Jnk, and p38 MAP kinases and the release of TNF-alpha. In contrast, decreased expression of Syk dramatically reduced FcepsilonRI-induced degranulation, activation of NFAT and NFkappaB. Therefore, the reduction in expression of these proteins in mast cells indicates that Syk is the major regulator of FcepsilonRI-mediated reactions, whereas Fyn has minor if any effects and Gab2 regulates primarily late events including MAP kinase activation and release of cytokines.