INDUCTION OF TOLERANCE BY PARABIOSIS 337 



* white form" observed in a previously used combination where 

 anaemia was the prevalent symptom. 



Despite the presensitization none of the Y59 grafts in Wistar 

 parabionts was rejected by the second-set reaction ("white 

 graft") and the mean survival time of grafts was found even to be 

 somewhat prolonged (lo- 8 ± 1-7 days). That the pretreatment of 



Table IV 

 Attempt to increase the incidence of tolerance in Y59 parabionts by 



PREIMMUNIZATION OF WiSTAR PARTNERS* 



Sunnval times ofhomografts Mean expectation of life 



Parabiont {days) (+ standard deviation) 



Wistar 7,9,5x10,3x11,3x12,13,14 io-8±i*7 



^ r8,9,2Xio,2Xii, 12, 13,14, 16,1 



^^9 |>,8t,i8,>3it,>33t,>4it J ^'^ 



* Immunization by i.P. injection of spleen cell suspension from Y59 donors five days 

 before operation for parabiosis and cross-grafting. 



t Died of "parabiotic disease" with surviving cross-graft. 



Wistar rats has, in spite of the above fmding, been effective in 

 sensitizing the recipients is revealed by the induction of fatal 

 "parabiotic disease" in a strain combination where it normally 

 is not induced. Preliminary experiments indicate that the masking 

 of the immune state in Wistar parabionts might have been caused 

 by parabiotic trauma, although we have failed to show the effect 

 of parabiotic trauma on survival time of first-set grafts (Nakic and 

 Silobrcic, 1962). 



Ghittierism 



Tolerant parabionts were tested for chimerism by the method 

 based on Mitchison's design as described by Billingham and 

 Brent (1959). Secondary hosts first received i.p. spleen or lymph- 

 node cell suspension from the tolerant parabiont and four to five 

 days later a skin graft from the parabiotic partner of the tolerant 

 animal. The presence of donor cells is shown by the fmding of 



