Natural cytotoxicity: early killing of allogeneic lymphocytes in rats

The early days of NK cells: an example of how a phenomenon led to detection of a novel immune receptor system - lessons from a rat model

In this review, I summarize some of the early research on NK cell biology and function that led to the discovery of a totally new receptor system for polymorphic MHC class I molecules. That NK cells both could recognize and kill tumor cells but also normal hematopoietic cells through expression of MHC class I molecules found a unifying explanation in the “missing self” hypothesis. This initiated a whole new area of leukocyte receptor research. The common underlying mechanism was that NK cells expressed receptors that were inhibited by recognition of unmodified “self” MHC-I molecules. This could explain both the killing of tumor cells with poor expression of MHC-I molecules and hybrid resistance, i.e., that F1 hybrid mice sometimes could reject parental bone marrow cells. However, a contrasting phenomenon termed allogeneic lymphocyte cytotoxicity in rats gave strong evidence that some of these receptors were activated rather than inhibited by recognition of polymorphic MHC-I. This was soon followed by molecular identification of both inhibitory and stimulatory Ly49 receptors in mice and rats and killer cell immunoglobulin-like receptors in humans that could be either inhibited or activated when recognizing their cognate MHC-I ligand. Since most of these receptors now have been molecularly characterized, their ligands and the intracellular pathways leading to activation or inhibition identified, we still lack a more complete understanding of how the repertoire of activating and inhibitory receptors is formed and how interactions between these receptors for MHC-I molecules on a single NK cell are integrated to generate a productive immune response. Although several NK receptor systems have been characterized that recognize MHC-I or MHC-like molecules, I here concentrate on the repertoires of NK receptors encoded by the natural killer cell gene complex and designed to recognize polymorphic MHC-I molecules in rodents, i.e., Ly49 (KLRA) receptors.

Partial NK cell tolerance induced by radioresistant host cells in rats transplanted with MHC-mismatched bone marrow

We have studied the effect of radioresistant host cells in inducing tolerance and adaptation of the MHC recognition repertoire of donor-derived NK cells in stem cell allotransplanted (allo-SCT) rats. Sub-lethally irradiated PVG.1AV1 rats (RT1(av1)) were transplanted with bone marrow from fully MHC-mismatched allotype-marked PVG.7B (RT1(c)) rats; MHC-identical PVG (RT1(c)) controls were transplanted in parallel. In the PVG.7B → PVG.1AV1 allogeneic chimeras, NK cells were donor derived and showed partial tolerance toward host cells. Allogeneic chimeras failed to efficiently reject PVG.1AV1 cells by an NK-mediated mechanism in vivo (allogeneic lymphocyte cytotoxicity), and IL-2-cultured NK cells derived from these chimeras showed diminished cytolytic activity against PVG.1AV1 cells in vitro. There were corresponding changes in the phenotype and function of the highly alloreactive Ly49i2(+) NK cells, which are specifically inhibited by a donor MHC class I ligand, RT1-A1(c). The ligand-negative host MHC haplotype apparently induced expression of a second uncharacterized inhibitory MHC receptor responsible for the partial tolerance toward host-derived cells, along with a modest increase in Ly49i2 receptor levels. The host MHC haplotype did not induce a general hyporesponsiveness in Ly49i2(+) NK cells, which showed normal activation responses in a panel of MHC congenic strains. The data suggest that the MHC constitution of radiation-resistant host cells can have permanent, albeit not fully tolerogenic, effects on the development of a functional NK repertoire following allo-SCT.

NK cell receptors in rodents and cattle

Natural killer (NK) cells discriminate between normal syngeneic cells and infected, neoplastic or MHC-disparate allogeneic cells. The reactivity of NK cells appears to be regulated by a balance between activating receptors that recognize non-self or altered self, and inhibitory receptors recognizing normal, self-encoded MHC class I molecules. Subfamilies of NK receptors undergo rapid evolution, and appear to co-evolve with the MHC. We here review present views on the evolution and function of NK cell receptors, with an emphasis on knowledge gained in cattle and rodents.

The effect of in vivo depletion of NKR-P1+ or CD8+ lymphocytes on the acute rejection of allogeneic lymphocytes (ALC) in the rat

We have depleted lymphocyte subsets in PVG and AO rats with MoAbs 3.2.3 (against NKR-P1 on NK and NK/T cells) and OX-8 (against CD8 on CTL and NK cells), and examined the effect on the killing of YAC-1 target cells in vitro and the effect on the acute rejection of small allogeneic lymphocytes in vivo (allogeneic lymphocyte cytotoxicity, ALC). While 3.2.3 treatment led to only a partial depletion of 3.2.3-positive cells in PVG rats, this treatment drastically reduced the number of NKR-P1+ cells in AO rats, abolished splenic NK activity against the NK-sensitive tumour target YAC-1, and markedly diminished the ALC response. Rats treated with OX-8 for 1 day showed a similar loss of NK cell function in vivo and in vitro. However, in rats treated with OX-8 for 3 days a 3.2.3+ and OX-8- population consisting of NK cells appeared, restoring ALC. The results demonstrate that NK cell responses can be greatly diminished after in vivo treatment with these MoAbs. Furthermore, they demonstrate that ALC is not necessarily linked to expression of the CD8 molecule.

Chimerism in peripheral blood of sensitized patients waiting for renal transplantation: clinical implications

BACKGROUND

Potential renal transplant recipients with preformed antibodies to HLA resulting from previous transplants, pregnancy, and/or transfusions are unlikely to receive an allograft. The factors contributing to the long-term maintenance of antibody titers in these individuals are still unknown. In the present study, we sought to determine whether chimerism in the blood correlates with maintenance of HLA sensitization in highly sensitized patients.

METHODS

Qualitative analysis of chimerism in blood of sensitized patients was assessed by polymerase chain reaction-sequence specific oligonucleotide probes (PCR-SSOP) to HLA-DR. PCR single-strand conformation polymorphism (PCR-SSCP) was used to confirm the extra HLA-DR antigens detected by PCR-SSOP.

RESULTS

Fourteen of 36 patients (38.9%) were positive for more than two HLA-DR indicative of chimerism. The presence of extra HLA-DR was confirmed by PCR-SSCP. When patients were analyzed on the basis of their panel-reactive antibody (PRA) status, 10 of 15 (66.7%) were positive for chimerism in the sensitized group, compared with only two of eight (25%) in the unsensitized group. Of the five males in the sensitized group who had received a blood transfusion but not a transplant, three were positive for chimerism. An association was observed between chimerism and maintenance of sensitization. None of the eight normal subjects studied demonstrated chimerism.

CONCLUSIONS

The results obtained with sensitized patients suggest an association between blood chimerism and maintenance of HLA sensitization. We speculate that chimerism may lead to long-term maintenance of anti-HLA antibody titers. This finding implies that abolition of chimerism could result in the eventual elimination of antigenic stimuli for antibody production against HLA antigens.

Interactions of allogeneic human mononuclear cells in the two-way mixed leucocyte culture (MLC): influence of cell numbers, subpopulations and cyclosporin

With organ allografts considerable numbers of donor-type mononuclear cells are transferred to the recipient, leading to bilateral immunological interactions between donor and recipient lymphocytes. To study such bilateral immune reactions in detail, human two-way MLC were performed. In this model proliferation kinetics, patterns of activation, and survival of the two populations were analysed, and the relevance of initial cell subset composition, relative cell numbers, and the effect of immunosuppression on this co-culture were evaluated. It could be demonstrated that with an initial 50:50 ratio of two populations of allogeneic cells one population dominated after 21 days of co-culture in 78 out of 80 combinations (97%) tested; the other population decreased markedly after an initially stable phase of 6-7 days. With unequal starting conditions the larger population dominated when resting cells were used, but small populations of preactivated cells or separated CD8+ cells could also dominate. Depletion of CD16+ natural killer (NK) cells and of CD2- cells (B cell and monocytes) had no effect on domination. Addition of cyclosporin delayed or blocked the domination process while addition of IL-2 accelerated it. Disappearance of one population was associated with detection of apoptotic cells. The findings indicate that co-cultures of allogeneic mononuclear cells are generally not stable for more than 1 week, but lead to active elimination of one population. CD8+ cells and particularly preactivated cells seem to play the most important role in that process, while NK cells are of less importance. Cyclosporin can prolong survival of allogeneic cells in co-culture. These observations suggest that under the conditions of clinical organ transplantation even small amounts of immunocompetent donor cells transferred by the graft may persist for some time and may, thereby, have the chance to exert immunomodulatory functions.

Positive and negative MHC class I recognition by rat NK cells

The prompt rejection of transplanted allogeneic lymphocytes by rat NK cells in non-sensitized recipients (allogeneic lymphocyte cytotoxicity or ALC) is determined by MHC genes as well as by genes located in the NK complex. The same genetic control is found when NK alloreactivity is measured by an in vitro assay, and we have employed this assay to delineate the specificity of NK cells for the MHC. The MHC of the rat, RT1, contains class I genes situated on either side of the class II/class III region. The majority of these class I genes are located in the RT1.C region and expressed class I products usually behave as non-classical (class Ib) molecules. They do not serve as restriction elements for the vast majority of conventional alpha/beta T-cells, in contrast to those class I molecules encoded by one or more loci in the classical (class Ia) region, RT1.A. However, NK cells appear to recognize the products of either class I region. Immunogenetic studies suggest that NK cells are inhibited by RT1. A molecules, whereas RT1.C region molecules may have a dual role in regulating NK cytolytic activity, i.e. they either inhibit or activate natural killing. Based on these premises, a model is proposed in which identification of a target as self or non-self depends on different receptors for class I in single NK cells, interpreting coincident positive and negative signals from the various target class I molecules. The putative role of peptides presented by class I, the biological implications, and the evolution of the NK receptors and their ligands are discussed.

Neonatally tolerant rats actively eliminate donor-specific lymphocytes despite persistent chimerism

Rats from the allotype-marked PVG-RT7b and PVG-RT1u-RT7b strains were injected at birth with semi-allogenic F1 bone marrow (BM) cells from athymic nude rats (PVG-rnu/rnu x PVG-RT1u-rnu/rnu) to induce neonatal tolerance. As adults, 97% of the animals accepted donor-specific allogeneic skin grafts and a majority (65%) of rats were chimeric, expressing the major histocompatibility complex class I and allotype marker of the donor strain. Similar results were obtained when PVG-RT1u-RT7b rats were injected at birth with fully allogeneic PVG-rnu/rnu nude BM cells: as adults, 94% accepted donor-specific skin allografts and 76% of recipients were chimeric. Donor derived CD4 T cells, CD8 T cells and B cells were found in low numbers (less than 2%) in peripheral blood of rats made tolerant by F1 BM cells. A large proportion of T cells bore the phenotype of recent thymic emigrants, suggesting that they were newly produced. All the evidence was consistent with clonal deletion tolerance, induced centrally within the thymus. The thymus was chimeric and thymocytes failed to respond in vitro to alloantigens of the donor-specific haplotype; donor-specific skin allografts survived indefinitely on athymic nude recipients reconstituted with CD4+CD8- thymocytes or peripheral CD4 T cells from tolerant animals. The chimeric state was interesting, since the PVG and PVG-RT1u rat strains contain a natural killer (NK) cell system that rapidly eliminates (within 24 h) intravenously injected allogeneic or semi-allogeneic lymphocytes--a phenomenon known as allogeneic lymphocyte cytotoxicity or ALC. When neonatal tolerant rats were tested, the ALC index (a measure of cell killing) was unchanged in nonchimeric tolerant rats and significantly altered (reduced killing), but not abolished in chimeric animals. Hence, the injection of allogeneic BM cells which induced specific tolerance in the T cell population failed to tolerize the NK cell system, allowing the constant killing of newly produced donor-derived lymphocytes and putting at risk the very survival of the allogenic BM cells. This has interesting implications for clinical transplantation.

Allogeneic lymphocyte chimerism after clinical lung transplantation

Donor lungs contain large amounts of passenger leukocytes which are transferred to the recipient by organ transplantation. In this study we have analysed the fate of these cells and have studied the populations of donor leucocytes detectable in the blood circulation of ten lung transplanted patients during the first postoperative weeks. To this aim we have applied immunocytological as well as flow cytometric analyses using monoclonal antibodies against polymorphic HLA class I antigens that differed between donor and recipient as well as antibodies against cell differentiation markers. The results demonstrate that donor cells can be detected in the circulation of all lung transplanted patients but there is a considerable interindividual variability between 0.9% and 17.5% (mean 5.1%) on postoperative day 3. Cells were usually detectable for 2-4 weeks and had disappeared in all patients after 1 month. The circulating donor cells consisted exclusively of lymphocytes. T cells were the predominant population, most of which seemed to be CD45R0+, but B and NK (natural killer) cells were also present. Probably due to the small numbers of patients studied no correlation between clinical parameters and the extent of donor lymphocyte persistence; there were no clinical graft-versus-host reactions. The findings demonstrate the regular existence of a transient (macro)chimerism due to passenger lymphocytes in the early phase after lung transplantation. The immunological function and the relation between this phenomenon and the long-term microchimerism which frequently develops after solid organ transplantation remain unclear.

Control of rat natural killer cell-mediated allorecognition by a major histocompatibility complex region encoding nonclassical class I antigens

The ability of natural killer (NK) cells to eliminate normal allogeneic hemic cells is well established in several species including mice, rats, and humans. The controlling elements for NK susceptibility in these species map to the major histocompatibility complex (MHC), but in contrast to findings in mice and humans, the mode of inheritance is not always recessive in rats. This finding is not easily explained by the missing self and hemopoietic histocompatibility (Hh) models for NK recognition, and has led to the idea that certain alloantigens may trigger NK cell reactivity. In our in vitro system for assessing rat NK alloreactivity, we have employed target and inhibitor cells from a large panel of MHC congenic, intra-MHC recombinant and MHC mutant rat strains, as well as appropriate F1 hybrids between them, and we show the following: (a) The nonclassical class I (RT1.C) region was most important in determining the susceptibility of target cells to alloreactive NK cells in vitro. Lymphocyte susceptibility to lysis in vivo also mapped to the C region, which supports the concept that the in vivo and in vitro alloreactivity assays reflect the same recognition process. (b) Four different RT1-controlled NK allospecificities (represented by the u, l, a, and n haplotypes) could be discerned when we used polyclonal NK cells from the PVG (RT1c) strain as effector cells. Three of the target specificities recognized were controlled mainly by the RT1.C region. (c) The expression of RT1.C region-controlled parental strain NK allodeterminants could be demonstrated in F1 hybrids heterozygous for the C region alone and were therefore inherited nonrecessively. (d) Loss of an RT1.C region-controlled NK allospecificity could be shown with the MHC mutant LEW.1LM1 rat strain characterized by a genomic deletion of about 100 kb of the C region. Taken together, these observations have demonstrated a major importance of the nonclassical class I region, i.e., RT1.C, in controlling rat NK allorecognition, and have thereby assigned a hitherto undescribed immunological property to this region. Furthermore, some of the present data are consistent with the existence of polymorphic NK-triggering alloantigens that are coded for by the RT1.C region.

Effect of anti-ganglioside antibodies on the metastatic spread of intraocular melanomas in a nude mouse model of human uveal melanoma

In vivo and in vitro studies were performed to determine: (a) if human uveal melanoma cells expressed GD2 and GD3 gangliosides; (b) if anti-GD2 monoclonal antibodies would inhibit the propensity of human uveal melanoma cells to localize in the liver following intravenous injection; and (c) if anti-GD2 monoclonal antibody would reduce the spontaneous metastasis of primary intraocular melanomas in nude mice. The results showed that all three of the human uveal melanoma cell lines tested expressed GD2 and GD3 gangliosides in vitro and in vivo. The human uveal melanoma cell lines preferentially localized in the liver and entered the hepatic parenchyma following spontaneous metastasis from the eyes of nude mice. In vivo administration of anti-GD2 monoclonal antibody produced a sharp reduction in the number of uveal melanoma cells that disseminated to the liver following either intravenous injection or by spontaneous metastasis from primary intraocular melanomas. Collectively, the results demonstrate that uveal melanoma cells display a propensity to localize in the liver after entering the bloodstream; however, this localization can be significantly inhibited by in vivo administration of anti-ganglioside antibodies. The expression of GD2 and GD3 surface gangliosides on uveal melanomas and the capacity of anti-ganglioside antibodies to inhibit metastasis formation in mouse models of ocular and cutaneous melanomas raise the possibility of implementing anti-ganglioside antibodies as potential therapeutic agents for the management of uveal melanoma.

Proliferation of Theileria annulata and Theileria parva macroschizont-infected bovine cells in scid mice

Theileria annulata and Theileria parva macroschizont-infected bovine cells formed tumours at the inoculation site when injected subcutaneously into C.B.-17 scid mice. T. annulata tumours showed more vigorous growth than T. parva tumours. The tumours did not regress and infected cells spread to other tissues. Intraperitoneal injection of high doses of T. annulata-infected cells resulted in the development of ascites: the infected cells colonized abdominal organs, in particular mesenteric tissue. Low doses of cells did not establish when administered by this route. Evidence for a role for macrophages in controlling proliferation of Theileria-infected cells was provided by finding (i) that uninfected bovine cells did not survive for as long in the peritoneal cavities of scid mice as in Balb/c mice: (ii) peritoneal macrophages both proliferated in vivo in the presence of infected cells and were activated as assessed by production of interleukin-1. Evidence against a role for NK cells was provided by (i) the failure of an in vivo assay for allogeneic lymphocyte cytotoxicity to reveal any activity against bovine cells in the lungs or liver, i.e. the sites usually associated with NK cell cytotoxicity, and (ii) the lack of correlation between tumour regression and NK cell activity in the spleens of mice with chronic T. annulata tumours.

Glomerulopathy Induced by Graft-Versus-Host Reaction in the Rat. Requirement of Donor CD4+ T Lymphocytes and MHC Class II Incompatibility at the Lymphoid Compartment

Graft-versus-host reactions (GVHR) can be associated with several autoimmune phenomena involving the kidney as a target organ. By transferring lymphocytes of AO rats into complete Freund's adjuvant-pretreated (AO x BN)F1 hybrids, a dose-dependent GVHR with glomerulopathy was experimentally induced. IgM, IgG1, and IgG2a were deposited in the mesangial area and along the glomerular basement membrane. Eluted immunoglobulins from diseased kidneys bound to normal basement membranes and especially to laminin. Anti-laminin reactivity was also present in sera from F1 recipients with GVHR. Parental CD4+ T lymphocytes were required and sufficient to induce GVHR and glomerulopathy in sublethally irradiated F1 hybrids. Using various F1 hybrids, MHC class II incompatibility was shown to be required for the induction of GVHR-associated glomerulopathy. Across MHC class I incompatibility, GVHR without glomerulopathy could be induced, provided that both CD4+ and CD8+ donor T lymphocytes were administered. Finally, MHC incompatibility between donor T lymphocytes and the recipient non-lymphoid compartment was found to be sufficient for the induction of GVHR, but not for GVHR-associated glomerulopathy. The results indicate that alloreactive donor CD4+ T lymphocytes have to interact directly with MHC class II alloantigen bearing host B lymphocytes in order to stimulate the latter to produce (auto-)antibodies. GVHR-induced glomerulopathy shares several immunopathological features with HgCl2-induced autoimmune glomerulopathy in the rat.

Allospecific recognition of hemic cells in vitro by natural killer cells from athymic rats: evidence that allodeterminants coded for by single major histocompatibility complex haplotypes are recognized

We have previously shown that large granular lymphocyte (LGL)-enriched cell populations have the capacity to spontaneously recognize and kill allogeneic small lymphocytes and bone marrow cells (BMC) in vitro in certain strain combinations of rats. Here, we have studied the alloreactivity of natural killer (NK) cells from PVG nude (RT1c) rats against a panel of major histocompatibility complex (MHC) incompatible hemic cells. Both lymphocytes and BMC from the AO (RT1u), DA (RT1a), BN (RT1n) as well as the MHC-congenic PVG-RT1u (RT1u) rat strains were efficiently killed in vitro, whereas cells from syngeneic PVG rats were spared. The structures recognized on lymphocytes and BMC were probably similar since the two cell populations inhibited each other in cross-competition experiments. A number of features aligned the alloreactive effector cells with NK cells and not T cells. (a) Only about 5% of the effector cells from nude spleens expressed the T cell antigens CD3, CD5 or T cell receptor (TcR) alpha/beta whereas greater than 50% of the cells expressed markers present on NK cells (CD2, CD8, OX52 and the rat NK cell-specific marker NKR-P1 recognized by the monoclonal antibody 3.2.3). (b) The alloreactive cells were granular since pretreatment of nude spleen cells with the lysosomotropic agent L-leucine methyl ester which eliminated LGL, simultaneously abolished the cytolysis of both allogeneic lymphocytes and YAC-1 tumor cells. (c) Nude spleen cells stimulated with human recombinant interleukin 2 for 1 week in vitro generated large granular proliferating cells which were CD3-, CD5-, TcR alpha/beta-, but greater than 95% 3.2.3+. These cells efficiently killed allogeneic hemic cells from the same rat strains as did freshly isolated effector cells. (d) The cytolysis of allogeneic hemic cells could effectively be inhibited with unlabelled NK-sensitive (YAC-1 and K-562), but not NK-resistant (Roser leukemia) tumor cells. Cross-competition studies showed that PVG nude NK cells discriminated between AO, BN and DA BMC, suggesting that different alloantigens were positively recognized by subsets of NK cells. The mode of inheritance of the allodeterminant specifically recognized on AO BMC was investigated in crosses and backcrosses between AO and BN or DA rats. A gene dosage effect was observed in that this determinant was expressed at a slightly reduced level in F1 hybrids.(ABSTRACT TRUNCATED AT 250 WORDS)

Splenectomy and renal allograft survival in the rat

The effect of splenectomy on renal allograft survival is not clear. In the rat, spleens isolated from recipients with functioning grafts have been shown to be a major source of cells that are capable of suppressing the rejection response (suppressor T lymphocytes). Thus the removal of the spleen in these allograft recipients could be detrimental to renal allograft survival. This study investigates this hypothesis, and looks for the presence of suppressor cells in other lymphoid organs apart from the spleen. In the rat renal allograft model, donor Lewis spleen cells given to DA recipients intravenously 1 week before transplantation of a Lewis kidney leads to indefinite allograft survival (median survival time (MST) greater than 100 days). Splenectomy before or after pretreatment with donor spleen cells failed to abrogate this effect (MST greater than 100 days). Experiments were performed in which cells or serum were prepared from long-term surviving splenectomized animals which had already been pretreated and transplanted, and then were injected into untreated recipients (adoptive transfer experiments). This was done to determine if cells capable of suppressing graft rejection were present in lymphoid organs outside the spleen in these splenectomized recipients. Thus the IV transfer of 10(8) lymph node cells harvested from splenectomized DA recipients with a long-term surviving LEW graft (LTS), into untreated but lightly irradiated (200 rad) DA recipients resulted in indefinite survival of a fresh Lewis kidney (MST greater than 100 days). In contrast, adoptive transfer of normal DA lymph node cells was ineffective (MST 13 days). Thus splenectomy is not necessarily detrimental to graft survival, as cells capable of preventing graft rejection are found in other lymphoid organs, such as lymph nodes, in splenectomized recipients.

Specific inhibition of natural killer (NK) activity against different alloantigens

Allogeneic lymphocyte cytotoxicity (ALC), i.e., rapid rejection of i.v. injected allogeneic lymphocytes in unprimed hosts, is an example of NK activity. Apparently anomalous rejection patterns, such as acceptance of F1 hybrid cells by parental hosts and rejection of parental cells by F1 hybrid hosts in many strain combinations, would fit the hypothesis that the effector cells in ALC recognize the absence of certain self-molecules (passwords) rather than the presence of nonself determinants. However, cold target inhibition studies showed that ALC displays allospecificity: when a mixture of radiolabeled AO and DA cells were injected i.v. into euthymic or athymic PVG rats, adding a surplus of cold DA cells reduced killing only of labeled DA cells and vice versa. Furthermore, semiallogeneic cold target cells were ineffective in inhibiting elimination of fully allogeneic cells, which supports the argument against a modification of the hypothesis that self-determinants inhibit a postbinding stage of lysis. Finally, (DA x AO)F1 cells injected into (DA x PVG)F1 hosts were rapidly rejected, despite the fact that donor and host shared expressed DA determinants. In sum, our results show that a hypothesis based on inhibition of killing by self-determinants can only be sustained with extensive modifications, and favor the alternative mechanism that the effector cells positively recognize the presence of allospecific determinants on the target cell surface.

Analysis of the genetic control of lymphocyte positioning

A possible role for the major histocompatibility complex (MHC) in the localization of lymphocytes in different lymphoid organs was investigated using inbred mouse strains. Lymphocytes labelled with the intracellular fluorochrome Hoechst 33342 (H33342) were transfused intravenously (IV) into unimmunized mice and the distribution of these labelled lymphocytes examined. In some combinations (e.g. C57BL/6----CBA) 2 h after injection allogeneic lymphocytes accumulated in the region between the marginal zones and outer aspects of the white pulp of the spleen. In contrast, in syngeneic controls (e.g. CBA----CBA) the lymphocytes migrated normally into the while pulp. Similar results were obtained in Peyer's patches. Mapping studies in the spleen indicated that the failure to migrate normally is predominantly controlled by the MHC complex, although some non-MHC genes may play a role. In the case of the MHC the most definitive combination was BALB/c-H-2dm2 (H-2L deletion mutant) lymphocytes transfused into BALB/c recipients, the mutant lymphocytes failing to migrate normally and, therefore, implicating the H-2L region in the phenomenon. No differences in the viability of labelled lymphocytes at 6 and 24 h after injection into either syngeneic or allogeneic recipients suggests that the inability of cells to passage through lymphoid organs may represent inappropriate receptors rather than elimination of the allogeneic lymphocytes by natural killer cells (NK) as previously proposed.

The roles of interdigitating cells and natural killer cells in the rapid rejection of allogeneic lymphocytes

The fate of radiolabeled allogeneic thoracic duct lymphocytes injected into congenitally athymic, nude rats was followed by autoradiography and electron microscopy. The allogeneic cells entered the host lymphoid organs at a normal rate, but once inside the lymphoid tissue they were rapidly phagocytozed by interdigitating cells (IDC) situated in the lymph node paracortex and the splenic periarteriolar lymphoid sheaths. Neither host nor donor T cells were required to initiate phagocytosis, as purified donor B cells were also avidly ingested by the athymic host IDC. Compared with fully allogeneic cells phagocytosis of semiallogeneic donor cells was much less efficient. When natural killer cell activity was blocked by preinjecting the recipients with antibodies against natural killer cells (anti-asialo GM1 or MRC OX-8) phagocytosis of the allogeneic cells was strongly reduced. As IDC did not bind these antibodies, the finding indicates that natural killer cells were needed to discriminate between own and foreign lymphocytes and to kill the allogeneic cells, which were then ingested by surrounding IDC. This was further supported by the observation that dendritic, constitutively Ia+ cells from peripheral lymph, phenotypically identical to IDC, did not lyse or phagocytoze allogeneic lymphocytes in vitro.

The popliteal lymph node graft-versus-host (GvH) reaction in the rat: a useful model for studying cell interactions in the immune response?

Simonsen (1962), in his comprehensive review article on GvH reactions, rightly emphasized the great applicability this reaction had in immunological research. A broader implication of GvH reactions than was perhaps expected was evident when the complex interactions between the donor cells the host's immune system were detected. In this review article, one aspect of the donor-host cell interactions is discussed. Using the popliteal lymph node GvH reaction as a model, it has been shown that the hyperplastic response in the lymph node is due to an accumulation and proliferation of cells almost exclusively of host origin. B cells are preferentially activated, and although mitogenic factors are released into the circulation during systemic GvH reactions, the interaction between donor TH cells and host B cells is probably of a more specific nature, involving only those B cells expressing MHC antigens to which the donor cells are able to react. This will presumably lead to a panclonal activation of host B cells with antibody production against a wide variety of antigens. Athymic nude rats are often surprisingly resistant to GvH reactions induced by allogeneic T cells, and this resistance can now be satisfactorily explained in terms of natural resistance mechanisms. Sometimes F1 hybrid rats are also resistant to GvH reactions induced by parental cells, indicating that this resistance may be restricted by non-codominantly inherited genes.

Immunological consequences of dioctyltin dichloride (DOTC)-induced thymic injury

Inbred rats fed diets containing 75 ppm dioctyltin dichloride (DOTC) for 8 or 12 weeks demonstrated marked reduction in thymic weight. This reduction was accompanied by a decrease in the number of W3/25 positive phenotypic circulating lymphocytes and an impaired ability to respond to suboptimal mitogenic stimulation. Responsiveness to alloantigenic stimulation, as measured by mixed leucocyte reactions, was depressed in DOTC-fed animals whereas no effect on natural cytotoxicity in nonimmunized treated or control rats was evident. No difference was found in the ability of treated compared to untreated animals to mount specific antibody responses against sheep red blood cells, and no evidence was found to indicate toxicity to lymphocytes in vivo. The investigation showed that DOTC administration resulted in a selective immunodeficiency.

The organization of cell populations within lymph nodes: their origin, life history and functional relationships

The normal lymph node comprises a superficial cortex, a deep cortex or paracortex and a medulla. In each of these regions there are three kinds of spaces: an intralymphatic space, an intravascular space and an extravascular space or interstitium. Both the vascular endothelium and the lymphatic endothelium are specialized in these different regions. The cell types in lymph nodes comprise lymphoid cells, accessory or non-lymphoid cells and stromal cells, and within these cell types a number of different sub-types can now be identified by means of enzyme- and immunocytochemistry. Based predominantly on experimental studies, the origin, migratory patterns, localization, inter-relationships and interactions between these various cells are reviewed.

The "natural resistance" to bone marrow allografts in normal and athymic nude rats. Rapid cytotoxic reactions both in vivo and in vitro

To elucidate the rapid destruction of allogeneic bone marrow (BM) grafts in nonsensitized rats we have assessed: (a) the 21-h tissue localization of 51Cr-labeled allogeneic vs. syngeneic BM cells, (b) the ability of allogeneic BM cells to proliferate in the BM or irradiated recipients, (c) the survival of allogeneic vs. syngeneic BM cells in cell-impermeable diffusion chambers implanted into the peritoneal cavity of rats, and (d) the destruction of allogeneic vs. syngeneic labeled BM cells in vitro by effector cells from various lympho-myeloid tissues. Some allogeneic BM cells were destroyed within 24 h after transfer to athymic nude rats, thus demonstrating the thymus independence of the cytotoxicity. Furthermore, allogeneic BM cells also failed to proliferate in the BM of irradiated recipients. However, the survival of allogeneic BM cells was not impaired if they were sheltered from host cells within cell-impermeable diffusion chambers over a culture period of 4 days. But when the allogeneic BM cells were similarly cultured in hosts presensitized against the BM donor, a substantial reduction in BM cell survival was observed. The effector cells of allogeneic BM cytotoxicity (ABC) were present in the spleen because PVG-rnu spleen cells were highly effective in lyzing BM cells from the AO, but not the PVG strain in vitro after only 4 h of culture. Some ABC activity in vitro was also found for peripheral blood lymphocytes and lymph node cells, but not for BM cells themselves. Taken as a whole these data provide firm evidence that the "natural resistance" to BM allografts is basically different from conventional immune responses, and in most, but not all respects resembles natural killer activity.