QUANTITATIVE STUDIES ON THE BEHAVIOR OF SENSITIZED LYMPHOCYTES IN VITRO : I. RELATIONSHIP OF THE DEGREE OF DESTRUCTION OF HOMOLOGOUS TARGET CELLS TO THE NUMBER OF LYMPHOCYTES AND TO THE TIME OF CONTACT IN CULTURE AND CONSIDERATION OF THE EFFECTS OF ISOIMMUNE SERUM

Studies on the immunotherapy of runt disease in rats

Using rats of the Lewis and BN (Ag-B locus incompatible) isogenic strains, a comparative study has been made of the capacity to prevent or mitigate the development of runt disease with: (a) lymph node cell suspensions from normal adult BN rats, (b) node cells, or (c) serum from donors sensitized against Lewis tissue antigens, or (d) heterologous anti-lymphocyte serum (ALS) raised in rabbits against rat thymocytes. Following a standard intravenous or intraperitoneal inoculation of 20 x 10(6) Lewis node cells into neonatal BN hosts, there are cutaneous manifestations of runt disease within 11-15 days and death invariably takes place within 20 days. However, complete protection is afforded by administration of a similar number of normal BN node cells via a different vein, or admixed with the otherwise harmful Lewis node cells. However, timing of the administration was crucially important-precedence or delay by as little as 4 hr resulted in a great impairment of protection. When the inoculations of the two cell suspensions were separated by 24 hr, no protection was afforded. These and other observations suggested that a necessary condition for protection of the hosts by unsensitized isologous cells requires that they establish a prompt and intimate confrontation with the homologous target cells. At the same dosage level, suspensions of node cells from sensitized isologous donors were much more effective therapeutically, saving the lives of 92% of treated subjects when administered after a delay of 3 days, and of 19% when the delay was 4 or 5 days. Of the various immunotherapeutic agents studied, daily injections of 0.2 ml of isoantiserum gave the best results, and could totally reverse the course of the disease even when initiated at age 10-13 days and subjects already presented symptoms. ALS, although inferior to isoantiserum at the dosage levels tested, proved to be superior to sensitized isologous cells as a protective agent, since the initiation of daily injections after delays of 6 or 8 days were still effective. The observations that delayed treatments of infant rats with isoantisera or ALS resulted in complete recoveries sustain the thesis that the lesions responsible for the fatal outcome of runt diseases are not inflicted at a very early stage. The efficacy of both isoantisera and ALS as a means of inhibiting the progression of homologous disease also suggests that they may have therapeutic value in situations where this condition is encountered.

Cytotoxicity mediated by soluble antigen and lymphocytes in delayed hypersensitivity. I. Characterization of the phenomenon

In the presence of specific antigen, lymph node cells from inbred rats with delayed hypersensitivity to tuberculoprotein, bovine gammaglobulin, and egg albumin produced progressive destruction of monolayers of rat embryo fibroblasts in tissue culture, first apparent at 48 hr and maximal at 72 hr. The effect was specific and did not depend on a genetic difference between the lymph node cells and target cells. It required antigen concentrations equal to or greater than 1.25 microg/ml and lymphocyte: target cell ratios of approximately 10 or 20:1. It could be evaluated both by a plaquing technique and by cell enumeration with an electronic particle counter.

Quantitative studies on the mixed lymphocyte interaction in rats. 3. Kinetics of the response

The proliferative interaction of cultured rat lymphocytes of immunogenetically disparate origin-the mixed lymphocyte interaction-was employed as an experimental model to examine the initial stages of the immune response mechanism. Using mixed cultures of cells derived from parental strain and F(1) hybrid rats, in which only the parental lymphocytes respond, the following observations were made on the magnitude and kinetics of the reaction. After initiation of the cultures, there was a latent period of approximately 40 hours during which time no mitotic activity was detected. This inactive phase was followed by a period of proliferation in which previously nondividing cells entered the mitotic cycle for the first time. Activity in the cultures, as detected by incorporation of radioactive thymidine and measured by radioautography or scintillation spectrometry, increased exponentially with a doubling time (T2) of 9-10 hr. In this exponential proliferative phase, lasting approximately 100 hr, the dividing cells underwent a series of rapid sequential divisions with a generation time (Tc) of 8 hr, and few, if any, dropped out of the mitotic cycle. In addition to the cells which first entered mitosis at the beginning of the proliferative phase and then proceeded through multiple divisions, significant numbers of new, previously nondividing cells continued to enter the mitotic cycle during the entire exponential growth phase. The total number of these newly responsive, first division cells throughout the total culture period amounted to 1-3% of the original parental cell inoculum. This is a surprisingly large proportion of peripheral blood lymphocytes with demonstrable reactivity to a particular antigen system, if it is assumed that these first division cells in vitro are functionally related to the hypothetical antigen-sensitive cells which proliferate and differentiate into immunological effector cells. At present there is no entirely satisfactory explanation for this large number of reactive cells in the mixed lymphocyte interaction.

Quantitative studies on the mixed lymphocyte interaction in rats. II. Relationship of the proliferative response to the immunologic status of the donors

The influence of the immunologic status of the cell donors on the proliferative behavior of rat lymphocytes in the mixed lymphocyte interaction has been studied. Mixed cultures of cells from various parental and F(1) combinations having morphologically distinguishable sex chromosomes exhibited unidirectional proliferative reactivity. The mitotic figures were predominately of parental origin. Lymphocytes from donors made tolerant at birth to homologous transplantation isoantigens were specifically unreactive against cells bearing antigens of the tolerance inducing strain, but not to indifferent third party homologous lymphocytes. Cells from animals that had been surgically thymectomized at birth exhibited a markedly and sometimes totally diminished reactivity against homologous lymphocytes. Presensitization of the cell donors resulted in a curtailment of proliferative reactivity in cultures with cells bearing the immunizing antigens. This may reflect the destructive properties that lymphocytes from sensitized animals are known to possess. The results of these experiments show that the proliferative activity of lymphocytes in the mixed lymphocyte interaction accurately reflects the immunologic status of the cell donors, and these findings provide further support for the premise that the mixed lymphocyte interaction represents a primary immunologic response by cells in culture against homologous cells bearing histocompatibility antigens.

Cytotoxic effect of lymphocyte-antigen interaction in delayed hypersensitivity

Lymph node cells from inbred rats having delayed sensitivity to soluble proteins inhibit growth of syngeneic or allogeneic fibroblasts in the presence of specific antigen. A relation is suggested between this in vitro phenomenon and other systems believed to be specific manifestations of delayed hypersensitivity.

Local recognition of histocompatibility differences in skin grafts

Studies in rabbits of skin grafts tagged with tritiated thymidine indicate a greater proliferation of endothelial cells and fibroblasts at the site of an allograft than at that of an autograft as early as the first day after grafting. It appears, therefore, that allogeneic differences can be recognized and responded to locally almost at once. Labeled nuclear material is found to be transferred from the epithelial cells of skin grafts to host cells of the adjacent tissues. A mechanism therefore exists which might effect a local transfer of information on histocompatibility differences.

Quantitative studies on tissue transplantation immunity. VII. The normal lymphocyte transfer reaction

The normal lymphocyte transfer reaction (NLT reaction) is a cutaneous inflammatory episode of delayed onset that is aroused when living lymphocytes from one guinea-pig are injected into the skin of another, and it occurs only in those genetic situations that show it to be an immunological response of the transferred lymphocytes to antigens of the animals into which they are injected (§4). When the recipient guinea-pig is exposed to 600 r whole-body irradiation before transfer, to delay the onset of an immunological counter-attack on the transferred cells, the NLT reaction evolves in three phases spread over about 6 days (§3). These are: ( a ) the first inflammatory episode, of moderate intensity, which reaches its peak at 24 h and remains stationary for 2 further days; ( b ) the flare-up , starting between the third and fourth days and rising to a peak of violent intensity at about the sixth day; and ( c ) the fade-out , which is mainly due to an immunological recovery of the host. The same three components may be discerned when the transferred lymphocytes have been presensitized against the tissues of their future recipients, but the pitch of the reaction is much higher throughout and presensitized cells perform as strongly at 24 h as normal cells do when they reach the peak of their activity at 5 to 6 days (figure 2). In general, lymphocyte transfer reactions vary in intensity rather than in tempo: the weak reactions that occur where antigenic disparity is slight, or when relatively few cells are injected, differ from strong reactions—even the very strong reactions caused by presensitized cells—in their general pitch of intensity but not in the relative timing of the various episodes of the response. In terms of the power of a given number of cells to excite an NLT reaction, blood lymphocytes were two to five times more active, and thymocytes ten to twenty times less active, than lymph node cells (§5). Cells from lymph nodes caused to enlarge greatly by stimulation with human gamma globulin emulsified in Freund’s complete adjuvant were not more effective than normal cells in exciting an NLT reaction. Lymphoid cells from foetal or newborn mesenteric nodes gave bold and clear NLT reactions rising to a peak not lower than that achieved by the same number of adult lymphoid cells (§5). The NLT reaction lent itself very well to a study of inhibitorsof the immunological response. Immunosuppressive agents (§6) were applied, as appropriate, to the cell donor before transfer, to the lymphoid cells in transit, or to the recipient before, during or after transfer. Immunosuppressive agents did not in general affect the first inflammatory episode; with varying degrees of effectiveness they did, however, eliminate the flare-up. The two most effective agents in this respect (Methotrexate and cyclophosphamide) did not, however, oppose the immunological performance of pre sensitized cells at concentrations more than sufficient to eliminate the flare-up. It is therefore reasoned that conventional immunosuppressive agents do not affect any distinctively immunological activity of lymphoid cells: they merely prevent the multiplication of the cells activated in the first episode of the NLT reaction, and therefore the transformation of a ‘normal’ into a sensitized population. Reasons are given for thinking that the antigens which excite the NLT reaction belong to the homograft system and that the reaction as a whole can be construed as a homograft reaction in reverse (§7). The first inflammatory episode is interpreted as the outward design of a ‘recognition’ event, i.e. of a distinctively immunological process, unaccompanied by cell division and in no way dependent on it, that occurs when a lymphocyte is first engaged by an antigen of the homograft system and is committed to the evolution revealed outwardly by the flare-up. A quantal theory of the reaction is proposed, according to which the violent response given by normal lymphoid cells at the peak of their flare-up, and by presensitized cells from the outset, is simply an arithmetic multiple of the recognition process, i.e. it simply consists in more cells doing what a relatively small number does in the first inflammatory episode. Immunosuppressive agents act by preventing this multiplication. This interpretation implies that any 'true’ immunosuppressive agent which weakens or abolishes the first inflammatory episode must weaken the performance of a presensitized population to an exactly similar degree.