Studies on transplantation immunity. IV. Murine natural immunity to lymphoid cells in vivo

Immunogenic and tolerogenic cell death

The immune system is routinely exposed to dead cells during normal cell turnover, injury and infection. Mechanisms must exist to discriminate between different forms of cell death to correctly eliminate pathogens and promote healing while avoiding responses to self, which can result in autoimmunity. However, an effective immune response against host tissue is often needed to eliminate tumours following treatment with chemotherapeutic agents that trigger tumour cell death. Consequently, a central problem in immunology is to understand how the immune system determines whether cell death is immunogenic, tolerogenic or 'silent'.

Modification, by a poly I:C injection, of organ distribution of intravenously injected murine lymphoma cells and of blood coagulability

51Cr-or 111In-labelled murine lymphoma cells were injected IV in control and poly I:C-treated mice. The organ distribution (lung, spleen, liver) of radioactivity was measured 2 h after injection. The results showed that if cell injection was performed 1 day after poly I:C treatment, the modifications of organ distribution did not fit with the expectations from a reinforcement of the NK function in vivo. In NK-suppressed mice, poly I:C affected the distribution of radioactivity in spleen and liver in the same manner as in normal mice, suggesting that the action does not entirely depend on the NK system. Additionally to that, poly I:C injections affected coagulability of the plasma from treated mice, by prolonging the coagulation time. It is concluded that poly I:C exerts a complex action on circulation and fixation of lymphoma cells.

The rapid elimination of allogeneic lymphocytes: relationship to established mechanisms of immunity and to lymphocyte traffic

Allogeneic lymphocyte cytotoxicity (ALC) refers to the destruction of lymphocyte beginning within a few hours of intravenous injection into non-sensitized, allogeneic recipients. Usually this has been detected in rats and mice by comparing the localization of 51Cr-labelled lymphocytes in the tissues of allogeneic and syngeneic recipients. In a particular strain combination the existence of ALC is supported by deficient localization of allogeneic lymphocytes in the LN, lungs and blood mononuclear population and an excess of the label that had been associated with allogeneic cells in the lymph plasma, blood plasma and kidneys. As the destruction of the allogeneic cells occurs in the lymphatic tissues, especially the spleen, it is paradoxical that there is sometimes an excess of the label associated with allogeneic cells in the spleen but evidence is presented that most of the isotope is no longer associated with living cells in that organ at 24 h after transfer. The data cannot be explained by an altered distribution of allogeneic lymphocytes between different organs. Experiments on the early migration of lymphocytes from the blood of syngeneic and allogeneic recipients point unequivocally to the conclusion that the adhesion of lymphocytes to specialized vascular endothelium in LN and their consequent entry into LN does not require that the lymphocytes and the endothelial cells share MHC products. The characteristics of ALC stressed in this review include the following: 1) it is independent of T-cell activation, either of host T-cells or of donor T-cells: 2) B- and T-cells are about equally vulnerable to ALC; 3) it varies greatly between different strain combinations regardless of other indices of cellular and humoral immunity; 4) F1 hybrid donor cells are vulnerable but the effect is always less marked than with allogeneic cells; 5) ALC is less radio-sensitive than primary cellular or humoral immune responses; 6) adoptive transfer of ALC can be achieved with TDL from nude rats. The possible mechanism underlying ALC has been debated in terms of natural killer cells recognizing certain allo-antigens or alternatively pre-existing "natural" antibody with low affinity for allogeneic cells leading to their elimination by ADCC. The argument hinges on the necessity for antibody and cannot be resolved by current data.

Elimination of allogeneic lymphocytes by mice

Two major classes of response to allogeneic lymphocytes can be detected in mice in vivo, based on injecting them intravenously with 51Cr-labelled lymph node cells and examining them in a short term assay. A natural immunity discriminating between allogeneic and syngeneic lymphocytes is seen in the lymph nodes (and to a lesser extent, the spleen), which has such close similarities to natural cell-mediated responses of the NK class as thymic-independence and radioresistance. However, it has immunological specificity of a conventional kind, probably towards serologically determined K/D antigens. There is also an active immune response, produced by immunisation with dissociated lymphoid cells or allografting, which consists of three elements: an IgG opsonising alloantibody response, diverting circulating lymphocytes to the liver; an IgM opsonin, localising them to the spleen; and a cell-mediated serum-dependent elimination mechanism that destroys cells entering the lymph nodes and spleen. Dose-response curves for the primary response show evidence of high-dose paralysis of elimination. Dose-time-response results for the secondary show a variety of unique characteristics; evidence is presented to suggest that several aspects of the phenomenon betray primitive features retained from an earlier stage in the evolution of the immunological system.

Natural cytotoxicity: early killing of allogeneic lymphocytes in rats

In many strain combinations among inbred rats, intravenously injected 51Cr-labelled lymphocytes are destroyed in substantial numbers by unsensitized allogeneic hosts. Destruction of cells (referred to as natural cytotoxicity (NC)) occurs within a few hours of injection, and is characterised by a decreased accumulation of radioactivity in the lymph nodes and increased renal excretion of label by allogeneic hosts, as compared with the distribution of label in syngeneic recipients of the same cell suspension. An intact spleen is necessary for killing. The level of NC expressed is consistent for a given donor-host combination. Using arbitrary criteria to compare the levels of NC expressed by different donor-host combinations among inbred rats, 13 of 95 strain combinations have been shown to express high NC, 63 intermediate NC and 19 low NC. The level of NC expressed cannot be correlated with the extent to which donor and host differ in respect of known MHC genes. Segregation analysis has shown high NC to be controlled by at least 2 independently segregating genes, one of which is MHC-linked. It is possible to weaken or abrogate NC by the neonatal injection of bone-marrow cells from the donor strain, and to reverse this reduced reactivity by the injection of host strain lymphocytes. The substitution of either the donor (P1) or the host (P2) by the P1 X P2)F1 hybrid reduces or eliminates NC in strain combinations normally expressing high NC. It is currently uncertain whether NC can be augmented. In the single strain combination in which maturation has been studied, NC becomes evident during the 4th week of life and attains adult levels during the 6th-7th weeks. NC is at least partially radio-sensitive. Two groups of reactivities appear to be related to NC: (i) those which have been identified in the context of aberrant lymphocyte homing, and for which allogeneic lymphocytes are the targets; (ii) the group of natural resistance systems which includes NK cells, and whose reactivity is directed against a variety of other target cells.

Natural cytotoxicity in rats: radiation-induced changes in the early killing of allogeneic cells

In some strain combinations among inbred rats intravenously injected 51Cr-labeled lymphocytes are rapidly destroyed in substantial numbers by unsensitized allogeneic hosts. This phenomenon has been referred to as natural cytotoxicity (NC) and is characterized by decreased lymph node radioactivity, increased kidney and urine radioactivity, and to a lesser extent increased liver radioactivity in allogeneic hosts, when compared with the distribution of label in syngeneic recipients of the same cell suspension. A single exposure to 800 rad either 1 or 7 days before the injection of 51Cr-labeled lymphocytes effected a reduction in NC as defined by all the above parameters in a strain combination exhibiting high NC. The same dosage of radiation abolished NC in a strain combination exhibiting intermediate leads to low NC. Because NC was not always completely abolished, the phenomenon was held to be partially radiosensitive. An increased accumulation of 51Cr-labeled lymphocytes was seen in the lymph nodes of both syngeneic and allogeneic irradiated hosts when compared with nonirradiated controls, although the increase was greater in allogeneic than in syngeneic hosts. This increased colonization in the lymph nodes of irradiated hosts seemed unlikely to be due to an increase in the available "space' in the lymph nodes following irradiation.

The recirculation of T and B lymphocytes in the athymic, nude rat

The recirculation of lymphocytes through the tissues and their return to the blood were compared in nude and euthymic rats. Three approaches were used: the organ distribution of 15Cr-labelled lymphocytes from nude or euthymic donors at 24 h after injection; the compartmental distribution of B and T lymphocytes as assessed by autoradiography of the spleen, lymph nodes, and Peyer's patches; and the tempo of recirculation from blood to thoracic duct lymph as estimated by counting timed fractions of lymph from a recipient of labelled lymphocytes. The following conclusions were drawn: (1) The distribution of lymphocytes between organs and within organs is very similar in nude and euthymic recipients. In particular, B lymphocytes proceed normally to the follicular areas in the absence of T cells. (2) The recirculation from blood to lymph is delayed in nude rats. (3) For equal numbers of B and T cells injected intravenously about half as many B cells as T cells enter mesenteric and cervical lymph nodes, but approximately equal numbers of B and T cells enter the spleen and Peyer's patches.

In vivo elimination of allogeneic lymphocytes in normal and T-cell-deficient rats. Elimination does not require T cells

The distribution of allogeneic and syngeneic thoracic duct lymphocytes was studied over 24 h in normal and T-cell-deficient animals (thymectomized, irradiated rats, 'B rats', or congenitally athymic nude rats). Initial migration from blood was no less for allogenic than for syngeneic cells. After 24 h, however, a marked deficit of radioactively labelled allogeneic cells as compared with syngeneic cells was found in the lymphoid tissue, whereas the allogeneic isotope was recovered in a relatively greater amount in liver, kidneys, and cell-free plasma and lymph. Most of the allogeneic cells are evidently destroyed within the first 24 h and their isotope released into body fluids. Our studies also revealed this process to be even more evident in T-cell-depleted environments. Autoradiographic studies of recipient nude rat spleens showed that allogeneic cells were not found in the great number seen in syngeneic transfers, but a high grain density in the periarteriolar lymphocyte sheath area could be observed. Granula seemed to be predominantly located over large nonlymphoid cells. The elimination of allogeneic lymphocytes is therefore governed by mechanisms independent of an intact thymus and may be due to a cell population or factor more active in nude animals than in their non-nude littermates.

Maintenance of allogeneic cell recognition in allograft tolerant mice

The organ distribution of 51Cr-labelled lymph node cells following transfer to syngeneic, allogeneic and specifically tolerant allogeneic recipients were compared. Neonatal induction of transplantation tolerance in several donor-recipient combinations involving different H-2 haplotypes had no effect on the reduction of homing into lymph nodes and the values of the homing did not differ from those of the untreated allogeneic recipients. In both cases, they amounted to 45-55% of the syngeneic controls. Even those neonatally treated animals, which had not been rendered tolerant at birth and rejected test skin grafts, were not capable of removing the transferred allogeneic cells by a typical immune elimination. The results indicate that animals recognize allogeneic cells, irrespective of whether or not they reject test skin grafts.