Brightness/Contrast adjustment and cropping of images were performed using Adobe Photoshop 6.0. == ELISA == ELISA antibody pairs and requirements for IgA, IgG1, IgG2a, IgG2b, IgG3, IgM and IgE, as well as IFN-, IL-4, IL-17, TGF- and IL-6 were purchased from BD Pharmingen, and IL-5, IL-13, IL-10 and IL-23 from eBioscience. APRIL plays an important role in the regulation of cytokine production, and that APRIL-triggered signals contribute to arthritis. Blockade of APRIL thus may be a valuable adjunct in the treatment of rheumatoid arthritis (RA). Keywords:TNF, T cells, Cytokines, Autoimmunity == Introduction == APRIL, a proliferation inducing ligand, also called TNFSF13, TALL-2, TRDL-1 or CD256 is usually a member of TNF superfamily [1,2]. APRIL and B-cell activation factor of the TNF family (BAFF, also named TNFSF13B, BLyS, TALL-1, THANK, zTNF4 or CD257) share two receptors, the transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI, also called TNFRSF13B or CD267) and B cell maturation antigen (BCMA, also called TNFRSF17 or CD269) [2]. BAFF in addition specifically binds BAFF receptor (BAFF-R, also named TNFRSF13C, BR3 or CD268) [2]. Proteoglycans have been identified as APRIL-specific binding partners [3,4]. Recently, a poor binding of a shorter variant of APRIL to BAFF-R was found in the murine system [5]. Both APRIL and BAFF are expressed in monocytes, macrophages, dendritic cells, T cells, B cells, osteoclasts, as well as airway and intestinal epithelial cells [2,611]. APRIL was also found in tumor tissues [1]. TACI, BCMA, and BAFF-R, receptors for BAFF and APRIL, are expressed on B cells [12]. While BAFF-R is also found on T cells [2], reports on TACI expression on T cells showed conflicting results [13]. APRIL-specific binding partners are expressed on B cells, T cells, plasma cells, nonhematopoietic cell lines, and tumor cell lines [3,4]. Functions of APRIL in tumor development, B and T cell immunity and autoimmunity have been reported [1,2,48,1121]. Studies on APRIL function in T cells have focused on T cell co-stimulation, proliferation and survival [4,1315,20,21], however, the biological function of APRIL in T cell immunity remains unclear. It has been reported that APRIL/mice were viable with normal T- and B-cell development and antibody responses [14]. However, in a second report [15], APRIL/mice experienced impaired IgA class switch, increased numbers of CD44hiCD62LloCD4+effector/memory T cells, and increased IgG responses to T dependent antigens. To clarify these discrepancies, further characterization of APRIL/mice will be helpful. Whether APRIL participates in rheumatoid arthritis (RA) and other autoimmune diseases is usually unclear. Collagen induced arthritis (CIA) is an animal model for RA, and requires both humoral and cellular (CD4+T cell) immune responses [17,22]. Collagen type II (CII)-specific autoantibody of the IgG2 isotype is NVP-BGT226 crucial to initiate CIA [23,24]. In addition, IL-17 generating Th17 CD4+T cells are very important [2527]. A TACI-Fc fusion protein, blocking both BAFF and APRIL, substantially inhibited mouse CIA [17], indicating that APRIL and/or BAFF contribute to CIA. Comparing the effects of BCMA-Ig and BAFF-R-Ig, BAFF appeared to be a key factor for the progression of CIA in mice [7]. Accumulating evidence showing elevated APRIL or APRIL/BAFF heterotrimers in sera and elevated APRIL in joints of RA patients suggests that APRIL may also play a role in RA [18,19,28,29]. Clearly, to better distinguish the function of APRIL and BAFF in RA, CIA studies under APRIL deficient conditions are needed. As reported here, we show that APRIL/mice have a bias towards Th2 response, diminished susceptibility to arthritis, diminished CII specific IgG2a autoantibody NVP-BGT226 levels and IL-17 production, and reduced IgA levels, indicating that APRIL is an important factor in T cell cytokine regulation and in autoimmune arthritis. == Results == == Generation of APRIL/mice == APRIL deficient mice NVP-BGT226 were created by replacing MMP3 exons 25 and the majority of exon 1 and 6 of the APRIL gene with a neomycin cassette (Fig 1A) by homologous recombination in embryonal stem cells and blastocyst injection. Homozygous APRIL/mice showed total absence of APRIL mRNA in splenocytes (Fig. 1E) while mRNA for TNF-related poor inducer of apoptosis (Tweak, or TNFSF12) and Sentrin specific peptidase 3 (Senp3), two genes located 794 bp and 731 bp up- and down-stream, respectively, of APRIL were expressed.