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Introduction: Shiga toxin type 2 (Stx2) mediates pregnancy loss in rats in earlystage of pregnancy. In the present work we assayed an immunization strategy withthe Brucella lumazine synthase-Stx2Bsubunit chimera (BLS-Stx2B) to protect pregnant rats from Stx2 mediated early pregnancyloss.Methods: Adult female Sprague Dawely rats were subcutaneously immunized with 3doses of BLS-Stx2B on days 0, 15 and 30. No immunized mothers were injected withvehicle. Serum samples were collected one day before every booster and anti-Stx2serum antibodies were checked by ELISA. Stx2-neutralizing activity was evaluatedin Vero cells. Female rats were then mated with males and day 1 of gestationwas determined when spermatozoa was found in the vaginal smear. At day 8 ofgestation immunized mothers were divided into two groups, and challenged intraperitoneally(i.p) with Stx2 (sublethal, 0.5 ng Stx2/g bwt) or PBS (Control) respectively. Noimmunized mothers were injected i.p. with Stx2 (sublethal, 100% of pregnancyloss). Mothers were weighted daily until 10 days after injection. Progressionof pregnancy, number, weight and behavior of the pups at birth were analyzedand compared between groups. Results and Discussion: Immunized rats developed high specific neutralizing antibodytiters and mothers challenged with Stx2 gained weight as controls and deliveredpups at term. No immunized mothers showed a marked decrease in weight until 4days after injection and then recovered gradually but had 100% of fetalresorptions. No difference was observed in the number and weight of the pups atbirth from immunized rats challenged with Stx2 compared to controls. <!-- /* Font Definitions */@font-face{font-family:Arial;panose-1:2 11 6 4 2 2 2 2 2 4;mso-font-charset:0;mso-generic-font-family:auto;mso-font-pitch:variable;mso-font-signature:-536859905 -1073711037 9 0 511 0;}@font-face{font-family:"Cambria Math";panose-1:2 4 5 3 5 4 6 3 2 4;mso-font-charset:0;mso-generic-font-family:auto;mso-font-pitch:variable;mso-font-signature:-536870145 1107305727 0 0 415 0;}@font-face{font-family:Calibri;panose-1:2 15 5 2 2 2 4 3 2 4;mso-font-charset:0;mso-generic-font-family:auto;mso-font-pitch:variable;mso-font-signature:-520092929 1073786111 9 0 415 0;} /* Style Definitions */p.MsoNormal, li.MsoNormal, div.MsoNormal{mso-style-unhide:no;mso-style-qformat:yes;mso-style-parent:"";margin-top:0cm;margin-right:0cm;margin-bottom:10.0pt;margin-left:0cm;line-height:115%;mso-pagination:widow-orphan;font-size:11.0pt;font-family:Calibri;mso-fareast-font-family:Calibri;mso-bidi-font-family:"Times New Roman";mso-ansi-language:ES-AR;mso-fareast-language:EN-US;}.MsoChpDefault{mso-style-type:export-only;mso-default-props:yes;font-size:10.0pt;mso-ansi-font-size:10.0pt;mso-bidi-font-size:10.0pt;font-family:Calibri;mso-ascii-font-family:Calibri;mso-fareast-font-family:Calibri;mso-hansi-font-family:Calibri;}@page WordSection1{size:612.0pt 792.0pt;margin:70.85pt 3.0cm 70.85pt 3.0cm;mso-header-margin:36.0pt;mso-footer-margin:36.0pt;mso-paper-source:0;}div.WordSection1{page:WordSection1;}-Studies are in progress to evaluate if pups born from pre-immunized ratswere additionally protected against Stx2 toxicity.Implications: Our results of Stx2 mediated early pregnancy loss in rats may be anindication that this event could happen in humans and that an effectiveimmunization could be an effective strategy to prevent this outcome.