Rupert E. Billingham

Immune privilege in the testis. I. Basic parameters of allograft survival

The fate of solid tissue allografts--skin or parathyroid glands--implanted in the interstitial tissue of the testis was investigated using inbred rats. The results affirm that the testis is an immunologically privileged site despite its efficient lymphatic drainage. Skin allografts survived at least several days longer than orthotopic grafts of similar size, whether major histocompatibility complex (MHC)-compatible or MHC-incompatible, and failed to induce an alloantibody response in most recipients or to prime for secondary antibody responses on rechallenge. Further assessment employed parathyroid grafts that allowed appraisal of their function and its duration, by monitoring serum calcium levels. Most intratesticular MHC-incompatible parathyroids survived for at least twice as long as control grafts in nonprivileged sites, with a median survival time (MST) of 41 days--and one-third of the grafts functioned at 100 days. MHC-compatible grafts fared even better (MST of 60 days), some surviving more than 400 days. Most F1 hybrid grafts survived virtually indefinitely. Splenectomy 5-23 days prior to implantation had a beneficial rather than a detrimental effect on the privilege afforded intratesticular parathyroid allografts. Allograft rejection was accompanied by antibody production in only one-half the animals. Grafts that had undergone functional rejection at the time of recovery usually had an intense mononuclear cell infiltrate, and long-term surviving grafts displayed varying degrees of cellular infiltration among cords of healthy, functional chief cells. Accepted parathyroids were destroyed by active immunization of the host but were unaffected by passively administered alloantibodies. The possible mechanisms controlling graft rejection in this unique privileged site are discussed.

Immune privilege in the testis. II. Evaluation of potential local factors

The significance of several local factors in prolonging allograft survival in the rat testis has been investigated. The lower temperature of this organ was shown to have no effect, because parathyroid gland allografts implanted s.c. in the ear, which is similarly hypothermic, were rejected as promptly as in conventional sites. Conversely, testes secured in the abdominal cavity afforded privilege to grafts just as well as normal testes. With indirect immunofluorescence using a monoclonal antibody, it was shown that the testis interstitial tissue is well-endowed with cells bearing class II histocompatibility antigens, a prerequisite for antigen-processing capability. When intratesticular allograft recipients were pretreated with estrogen to suppress Leydig cell synthesis of testosterone, most grafts were rejected promptly, suggesting that local steroid secretion is important in testicular immune privilege. Both testes and s.c. sites supported inflammatory reactions to cotton pledgets, indicating that there is no interference with the nonspecific inflammatory process necessary for graft rejection, but more likely with antigen processing itself.

Maternally Acquired Runt Disease

Without altering the structural integrity of the placenta by irradiation or drugs, we have shown that it is possible to immunize females both adoptively and actively against the paternally inherited transplantation antigens of their fetuses. Such immunization causes a high incidence of runt disease among the litters. Although the putative chimeric status of the affected offspring has yet to be confirmed, the results of our experiments support the thesis that runt disease is caused by the activities of unwanted immigrant lymphocytes from the maternal circulation. Our results suggest that immunologically activated cells are more likely to cross the placenta than normal cells and that this greater mobility may not be related to the immunologic specificity of the activated cells. Two factors may have contributed to the apparent failure of numerous previous attempts to demonstrate the capacity of transplantation immunity to affect the well-being of a fetus or, more correctly, its placenta, in the way that might be expected of a homograft. (i) Investigators were preoccupied with obtaining a classic type of rejection, in utero, analogous to the rejection of an orthotopic skin homograft. The birth of consistently healthy-looking litters, interpreted as a failure of the experiment, convinced the investigators of the efficacy of natures solution of the homograft problem and there was no reason for them to suspect its possible limitations. Observation of the litters for several weeks might have uncovered the phenomenon of maternally induced runt disease. (ii) Most investigators resorted to hyperimmunization of the mothers. This would have facilitated the synthesis of protective isoantibodies capable of interfering with the expression of the potentially harmful cellular immune response (6). Ever since the abnormalities of runt disease were first described they have repeatedly been compared to those observed in patients with certain lymphomas (17). Various theories have been propounded as to how maternally transmitted graft-versus-host reactivity might lead to the development of these tumors. In mice it has been established that graft-versus-host reactivity may result in a high incidence of lymphomas (18). Recent analysis indicates that this graft-versus-host reactivity unmasks and activates normally latent and undemonstrable oncogenic viruses (19). The work we describe in this article may have some relevance to the possible clinical significance of transplacental cellular mobility in man. We suggest that the relatively high incidence of lymphomas in children might also be, in part at least, due to unmasking of oncogenic viruses by subclinical graft-versus-host reactivity mediated by immunocompetent cells of maternal origin. The statistical evidence that male infants are at greater risk than females (20) is concordant with our observation that maternally induced runts include a significantly higher proportion of males than females (10).

The cumulative effect of histocompatibility antigens.

SUMMARYIn order to study the cumulative effect of mouse histocompatibility antigens, donor-host combinations were utilized which differed at 2, 3, 4, and multiple histocompatibility loci. Double difference pairs involving the Y-linked and one of several autosomal loci were produced by grafting from

RECONSIDERATION OF THE LYMPHATIC DRAINAGE OF THE RAT TESTIS

An extensive investigation of the lymphatic drainage from the rat testis was carried out in an effort to detect any characteristics which might be important in the immunological privilege the testis extends to allografts. Three different methods revealed a consistent and very efficient lymphatic drainage involving primarily the iliac and renal lymph nodes and to a lesser extent the external lumbar, para-aortic, and posterior gastric nodes. Within minutes, dye or labeled cells injected into the testis could be found within regional nodes. Node hypertrophy was induced by injecting the testes of F1 hybrids with parental strain lymphoid cells (a regional graft-versus-host reaction). Besides indicating that efficient filtration occurs in these nodes, this also established that they are a hospitable environment for cellular immune reactions. Similarly, the capacity of allogeneic cells injected into the testis to induce production of humoral alloantibodies by recipients confirms that exposure to antigens via this route does not in itself suppress immune responsiveness.

Privileged status of the subcutaneous site for skin allografts in rats.

Evidence is presented that in rats the subcutaneous site can extend privilege to both major histocompatibility complex (MHC)-incompatible (FI X DA)F1 leads to FI and MHC-compatible LEW leads to FI skin allografts, approximately doubling the median survival time of similar grafts transplanted orthotopically. Unlike graft dosage, gene dosage was an important variable in that grafts from (FI X DA)F1 donors significantly outlived those from DA strain donors. Prior splenectomy of the hosts did not prejudice the capacity of their subcutaneous sites to extend privilege. It was found that the hemagglutinin response incited by subcutaneous grafts was significantly delayed compared with that evoked by similar grafts transplanted orthotopically or intraperitoneally. This observation, coupled with our inability to demonstrate the passage of India ink to regional lymph nodes after its injection into the dermis of established subcutaneous grafts of syngenic skin, is consistent with the concept that poor endowment of the subcutaneous milieu with both blood and lymph vessels is the principal factor underlying its hospitality to allografts.