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Journal of General Virology 1997 78 2483 2487 Printed in Great Britain SHORT COMMUNICATION In vivo induction of interferon a in pig by non infectious coronavirus tissue localization and in situ phenotypic characterization of interferon a producing cells Sabine Riffault Charles Carrat Lydia Besnardeau Claude La Bonnardie re and Bernard Charley Unite de Virologie et Immunologie Mole culaires INRA 78352 Jouy en Josas cedex France A low frequency peripheral blood mononuclear cell PBMC subpopulation referred to as natural inter feron producing NIP cells is described as pro ducing interferon a IFN a following contact with non infectious viral structures namely viral glyco proteins These cells are characterized in vitro as non T non B MHC class II M and CD4 M cells In this study NIP cells were analysed in vivo after an intravenous injection of UV inactivated transmiss ible gastroenteritis virus in newborn piglets which resulted in strong serum IFN a production Spleno cytes but not PBMC were the IFN a producers in vivo Using double immunohistochemical labelling for both IFN a and leukocyte markers we estab lished that splenic NIP cells were not T or B cells The majority were MHC class II M and only a minority expressed a macrophage marker NIP cells were localized in contact with MHC class II expressing cells and T cells which suggested that NIP cells might modulate the antiviral immune response Production of type I interferon IFN IFN a b constitutes one of the earliest host responses to viruses In addition to their well established antiviral effects Van den Broek et al 1995 type I IFN can also modulate antiviral immune responses Belardelli Nowacki Eloranta et al 1996 Riffault et al 1996 In our previous studies on IFN a induction we took advantage of several features of transmissible gastroenteritis virus TGEV in order to analyse the role played by external viral glycoproteins in inducing leukocytes to secrete IFN a in this virus model TGEV is an enveloped RNA coronavirus which induces acute and often fatal diarrhoea and high IFN a production in infected newborn piglets La Bonnardie re Riffault et al 1997 In addition a TGEV mutant named dm49 4 selected on the basis of its defective ability to induce IFN a production in vitro is characterized by a point mutation in the N terminal domain of gM Laude et al 1992 In order to analyse in vivo the mechanism of IFN a induction by non infectious TGEV and to identify the origin of IFN a SC colostrum deprived 24 h old piglets were injected i v with UV inactivated TGEV 1 5 10 p f u before UV inactivation Two TGEV strains the wild Purdue 115 strain and the mutant virus dm49 4 Laude et al 1992 were used as virus sources Blood was collected at 0 7 and 30 h post injection p i The kinetics of serum IFN a production was 0001 4767 1997 SGM CEID S Riffault and others Table 1 Frequency of IFN a SC in blood and lymphoid organs 7 h after inactivated TGEV i v injection Cells were either assayed directly after their isolation from tissue ex vivo or following in vitro restimulation with UV inactivated TGEV in vitro IFN a SC per 10 5 cells Tissue of origin of cells ex vivo in vitro Spleen 1 95 0 33 nfl11 10 6 2 4 nfl11 Blood 0 nfl12 19 2 4 2 nfl15 Mesenteric lymph node 0 nfl8 0 10 0 10 nfl10 Liver 0 25 0 25 nfl4 0 30 0 19 nfl7 IFN a SC were detected using a specific ELISPOT assay The number of nucleated cells in each cell suspension used in the ELISPOT assay was counted to calculate the frequency of IFN a SC and results were expressed as the number of spots per 10 residual IFN a titres 121 43 U ml nfl9 were still present at 30 h p i Similar IFN a production kinetics are described after i v injection of UV inactivated HSV in C57Bl 6 mice Bhuiya et al 1994 Eloranta et al 1996 Furthermore the rapid IFN a production described above was similar to the IFN a response of pigs experimentally infected with TGEV La Bonnardie re Nowacki et al 1993 This last finding and the fact that in vivo induction of IFN a SC was mediated by non infectious TGEV possibly by its gM strongly suggested that the IFN a SC described in the present study were the in vivo counterpart of the NIP cells previously described in vitro Because in vivo induced IFN a SC were almost exclusively detected among spleen mononuclear cells by ELISPOT their localization in spleen was studied using immunohistochemical staining Pieces of spleen were fixed in 10 formol in PBS dehydrated and finally embedded in paraffin 54 56 C Rabbit IgGs raised against purified recombinant porcine IFN a MPA1 Lefe vre et al 1990 were used to stain IFN a SC in paraffin spleen sections in Tris buffer 50 mM pH 7 4 with 0 02 saponin 0 2 CaCl and 1 heat inactivated normal porcine serum NPS Sections were then incubated with alkaline phosphatase conjugated goat anti rabbit IgG Sigma and the IFN a SC were visualized using the Fast Red substrate Fast Red TR Naphthol AS MX Sigma A few clearly positive IFN a SC mainly located in the periarteriolar lymphatic sheath were detected in spleen sections from UV inactivated TGEV injected animals Fig 1a The IFN a SC appeared as large cells whose cytoplasmic extensions were sometimes visible Fig 1b No staining was observed with non immune rabbit IgG or with sections from MEM injected animals not shown In CEIE Nature of IFN a producing cells a b c d h g f e Fig 1 Localization and phenotype of IFN a SC in spleen 7 h after i v injection of UV inactivated TGEV Immunohistochemical staining using anti poIFN a antibodies and paraffin sections showed IFN a SC in the periarteriolar lymphatic sheath a with abundant cytoplasm and cytoplasmic extensions b Double staining of spleen cryosections with anti poIFN a antibodies red and anti leukocyte antibodies dark blue showed that IFN a SC are distinct from CD3 expressing cells c and from Ig light chain expressing cells d Some IFN a SC did express MHC class II molecules e or a macrophage marker SWC3a g Conversely MHC class II molecules f or a macrophage marker SWC3a h were not detectable on some IFN a SC Original magnification 132 for a and 330 for b h contrast to the location of porcine IFN a SC the murine splenic IFN a SC induced by i v injection of UV inactivated HSV are exclusively found in the marginal zones Eloranta et al 1996 This discrepancy may reflect the fact that the spleen structure of newborn piglets is not fully differentiated The phenotype of NIP cells has until now only been CEIF S Riffault and others Table 2 Specificity of mouse MAbs directed against swine leukocytes used in this study MAb Specificity Isotype Origin MSA3 SLA DR MHC class II IgG2a Hammerberg see Kaeffer et al 1991 PPT3 CD3 IgG1 Yang Nowacki Svensson et al 1996 whereas the MHC class II or macrophage marker phenotypes might well be compatible with the hypothesis of a monocytic lineage Francis Grage Griebenow et al 1996 Previous studies have shown that IFN a SC are mainly detected in the lymphoid tissues such as the spleen or regional lymph nodes following non infectious virus injection Arturs son et al 1995 Splichal et al 1995 Eloranta et al 1996 Riffault et al 1996 However this is the first demonstration that circulating PBMC did not produce a detectable amount of IFN a in vivo One reasonable explanation might be the rapid trapping of virions in the spleen after UV inactivated virus i v injection Combined staining of NIP cells and leukocyte subpopu lations showed that NIP cells were in contact with MHC class II cells T cells and occasionally macrophages It is therefore conceivable that NIP cells could act as accessory cells at the time of antigen presentation to naive T cells Indeed IFN a is involved in the promotion of T cell differentiation towards the Th1 phenotype Belardelli Accepted 20 May 1997 CEIH