Natural and induced tolerance in an immune network model

Combining Theoretical and Experimental Techniques to Study Murine Heart Transplant Rejection

The quality of life of organ transplant recipients is compromised by complications associated with life-long immunosuppression, such as hypertension, diabetes, opportunistic infections, and cancer. Moreover, the absence of established tolerance to the transplanted tissues causes limited long-term graft survival rates. Thus, there is a great medical need to understand the complex immune system interactions that lead to transplant rejection so that novel and effective strategies of intervention that redirect the system toward transplant acceptance (while preserving overall immune competence) can be identified. This study implements a systems biology approach in which an experimentally based mathematical model is used to predict how alterations in the immune response influence the rejection of mouse heart transplants. Five stages of conventional mouse heart transplantation are modeled using a system of 13 ordinary differential equations that tracks populations of both innate and adaptive immunity as well as proxies for pro- and anti-inflammatory factors within the graft and a representative draining lymph node. The model correctly reproduces known experimental outcomes, such as indefinite survival of the graft in the absence of CD4⁺ T cells and quick rejection in the absence of CD8⁺ T cells. The model predicts that decreasing the translocation rate of effector cells from the lymph node to the graft delays transplant rejection. Increasing the starting number of quiescent regulatory T cells in the model yields a significant but somewhat limited protective effect on graft survival. Surprisingly, the model shows that a delayed appearance of alloreactive T cells has an impact on graft survival that does not correlate linearly with the time delay. This computational model represents one of the first comprehensive approaches toward simulating the many interacting components of the immune system. Despite some limitations, the model provides important suggestions of experimental investigations that could improve the understanding of rejection. Overall, the systems biology approach used here is a first step in predicting treatments and interventions that can induce transplant tolerance while preserving the capacity of the immune system to protect against legitimate pathogens.

The idiotypic network in the regulation of autoimmunity: Theoretical and experimental studies

The regulation of autoimmunity is a key issue in fundamental immunology. Despite outstanding achievements on this front, we currently have more questions than answers. The idea of an immune network as a regulatory mechanism is quite attractive, since it enables us to explain the selectivity (specificity), and moreover the clonality, of the regulation. Nevertheless it remains unclear how this mysterious network of immune cells is organized, how it operates, and how it exerts control over autoimmunity. This article presents an attempt to understand how the immune network functions and how it controls autoreactivity. We present a mathematical model of the immune network that is based on principles of immune network organization and function that we arrived at from a survey of the available literature. To test the principles on which the mathematical model is based, we studied the model and compared the different responses to antigen that it generated with the results obtained from experimental studies of immune response. The modeled kinetics of idiotype and anti-idiotype in response to the administration of antigen are in good agreement with the experimental kinetics of idiotypic and anti-idiotypic antibodies. To obtain evidence of the existence of idiotypic mechanisms for regulating autoimmunity, we studied a mathematical model containing autoclones and compared the model results with data from experimental studies in a model of autoimmune hemolytic anemia in mice. Because the results from the theoretical and the experimental studies coincide, there is justification to conclude that autoreactive lymphocytes are normal components of the immune network within which they are regulated. We discuss a possible molecular/cellular mechanism for negative control of autoreactive cells as affected by anti-idiotypic antibodies.

Rheumatoid factor in idiotypic regulation of autoimmunity

AIM

Rheumatoid factor (RF) is known to be heterogeneous, and RFs detected by various methods exhibit different characteristics. In addition to interacting with the Fc region of immunoglobulin G (IgG), certain RFs are able to recognize idiotypes of antibodies. Given the important role of idiotypic interactions in regulating autoimmunity, we hypothesize that RF is involved in regulation of lymphocyte activity against autoimmune disease-inducing antigens via idiotype-anti-idiotype interactions with these lymphocytes.

METHOD

RF level and the existence of idiotype-anti-idiotype interactions between RF and antibodies to autoimmunity-inducing antigens were studied in rats resistant and sensitive to collagen-induced arthritis, encephalomyelitis and atherosclerosis. RF was assayed by agglutination of tanned IgG-loaded erythrocytes.

RESULTS

Rat resistance to autoimmune disease is associated with high RF production during the initiation of the immune response, and a low RF level during this period may be a preclinical marker of experimental autoimmune disease manifestation. RF-containing sera compete with an antigen if the RF-containing sera were obtained from rats immunized with that antigen, and they non-specifically inhibit binding of different antigen-antibody pairs. This suggests that RFs are anti-idiotypic antibodies that carry two kinds of paratopes: a particular paratope that recognizes the antigen-binding sites of antibodies, and a shared paratope that serves to recognize the recurrent idiotype on antibodies. Antigenic epitopes for the shared RF paratope can be created in the hinge region of Fc fragments of homologous IgG.

CONCLUSION

RF detected by agglutination of tanned IgG-loaded erythrocytes is involved in negative idiotypic regulation of lymphocytes specific to autoimmunity-inducing antigens.

Self-tolerance in a minimal model of the idiotypic network

We consider the problem of self-tolerance in the frame of a minimalistic model of the idiotypic network. A node of this network represents a population of B-lymphocytes of the same idiotype, which is encoded by a bit string. The links of the network connect nodes with (nearly) complementary strings. The population of a node survives if the number of occupied neighbors is not too small and not too large. There is an influx of lymphocytes with random idiotype from the bone marrow. Previous investigations have shown that this system evolves toward highly organized architectures, where the nodes can be classified into groups according to their statistical properties. The building principles of these architectures can be analytically described and the statistical results of simulations agree very well with results of a modular mean-field theory. In this paper, we present simulation results for the case that one or several nodes, playing the role of self, are permanently occupied. These self nodes influence their linked neighbors, the autoreactive clones, but are themselves not affected by idiotypic interactions. We observe that the group structure of the architecture is very similar to the case without self antigen, but organized such that the neighbors of the self are only weakly occupied, thus providing self-tolerance. We also treat this situation in mean-field theory, which give results in good agreement with data from simulation. The model supports the view that autoreactive clones, which naturally occur also in healthy organisms are controlled by anti-idiotypic interactions, and could be helpful to understand network aspects of autoimmune disorders.