A second study on the H-Y transplantation antigen in mice

In mice, H-Y, a histocompatibility locus on the Y-chromosome, is responsible for the rejection of male skin isografts by females of many isogenic strains. As a histocompatibility locus H-Y is unique and important experimentally because of its apparent lack of alleles and its absence in females (section 1). Using mice of the C 57 BL/6 strain, where the MST of male to female grafts is 25 $\pm $ 2$\cdot $5 days, a comparison has been made of the various means by which adult females can be rendered unresponsive of the Y antigen (as revealed by inability to reject male skin isografts). The extent to which these various tolerance-inducing procedures result in the establishment of detectable levels of cell chimerism, and the susceptibility of these tolerant states to abrogation by transfer of isologous lymphoid cells have also been investigated. A high proportion of adult C 57 females were rendered tolerant of the Y antigen following: (a) Intravenous or intraperitoneal inoculation of very high dosages (at least 500 million) of isologous male spleen cells. The former route was the more effective, and repeated exposures to the antigenic stimulus were more effective than single ones (section 4.1). (b) The receipt of orthotopic skin grafts from infant C 57 males (section 4.2). (c) Multiparity by C 57 males (section 4.3). (d) About 3 weeks9 parabiotic union with isologous males, or isologous females which were themselves tolerant of the Y factor as a consequence of neonatal inoculation with isologous or even homologous (A strain) male cells (section 4.4). Parabiosis with females rendered tolerant through multiparity was ineffective. Furthermore, parabiosis of specifically sensitized females with isologous males did not affect their immunological status. Indeed, some of the male partners succumbed to 9parabiosis intoxication9. A sensitive test for chimerism was developed, capable of revealing the presence of as few as 12 500 Y-antigen-containing cells in a standard test population of 20 million spleen cells (section 6.1). On the basis of this test (section 6.2), females rendered tolerant by inoculation of isologous male cells at birth, or in adult life, were chimeric as were females neonatally inoculated with (CBA $\times $ C57)F$_{1}$ spleen cells. Chimerism was infrequent in females rendered tolerant by neonatal injection of homologous marrow cells. It was consistently demonstrable in females rendered tolerant by parabiosis with isologous males but not in females which had been made tolerant by parabiosis with females rendered unresponsive by neonatal injection with homologous (AU) marrow cells, or in females rendered tolerant by multiparity. These findings indicate that apparently similar states of tolerance of the Y factor can be induced in adult hosts by two widely disparate antigen dosage ranges (section 7.2), achieved by different regimens of administration or exposure. The lower effective dosage ranges do not result in the establishment of detectable levels of cell chimerism. Attempts to induce tolerance of the Y antigen by means of single or multiple, spaced intravenous or intraperitoneal injections of very low dosages (ranging from 7 $\times $ 10$^{3}$ to 2 $\times $ 10$^{6}$) of isologous male splenic cells were only marginally successful (section 4.5). The results of studies on the abolition of tolerance of the Y factor by transfer of isologous lymphoid cells from specifically sensitized females (section 5; section 7.5) indicated that a necessary condition for abrogation of tolerance was a demonstrable level of chimerism. Putatively non-chimeric or very low level chimeric tolerant females were unaffected by the intraperitoneal transfer of even three donor equivalents of immune cells. However, such animals could easily be made susceptible to adoptive immunization either by giving them a second isograft of male skin, or by increasing the number of Y-antigen-containing cells in their bodies by injecting them with 20 to 30 million isologous male spleen cells, at about the time of transfer of immune lymphoid cells. These findings suggest that when lymphoid cells are transferred from a sensitized to a tolerant, or probably even to a normal host, further antigenic stimulation is necessary to make them immunologically effective (section 7.5). Possibly the transferred cell population does not include enough 9ready-made9 cells of the type that can immediately circulate in the blood stream and infiltrate and mount an effective immunologic attack upon a well established 9target9 graft. The phenomenon of 9split-tolerance9 is discussed in relation to some new observations bearing upon it (section 7.4), which suggest that the phenomenon may apply to homologous as well as to F$_{1}$ hybrid cellular inocula.