The Immune System as a Complex Adaptive System: A RePast Simulation of the Anti-Viral Immune Response

1. The Immune System as a Complex Adaptive System: A RePast Simulation of the Anti-Viral Immune Response Virginia A. Folcik, Ph.D. vnivar@hotmail.com Charles G. Orosz, Ph.D. orosz-1@medctr.osu.edu The Department of Surgery/Transplant, The Ohio State University College of Medicine and Public Health

2. The Immune System as a Complex Adaptive System: A RePast Simulation of the Anti-Viral Immune Response. Virginia A. Folcik and Charles G. Orosz, Department of Surgery/Transplant, The Ohio State University College of Medicine and Public Health The immune system is a prime example of a complex adaptive system, with individual cells that follow rules for behavior based upon detection of signals and contacts with other cells in the environment. We have created a simulation of a human anti-viral immune response using the RePast software framework. The agent-based simulation includes three windows that represent a generic tissue site with parenchyma that becomes infected with virus, a lymph node site with cells that can become activated to fight the viral infection, and the peripheral blood that carries the responding immune cells and antibodies back to the site of infection. The simulation uses seven agent types and twenty signals to represent Parenchymal Cells, B-Cells, T-Cells, Macrophages, Dendritic Cells, Natural Killer Cells and the virus, and pro- and anti-inflammatory cytokines, chemokines and antibodies that such cells use to communicate with each other. The numbers of agents present as well as the quantity and types of signals present depend upon rules for proliferation and the release of cytokines that the agent types follow. Individual agents have various states, migrate from one window to another and live or die as the rules for their behavior dictate. A typical run of the simulation involves the entry of initial conditions (ratios of immune cell types), then the execution of the simulation during which the numbers of agents and quantities of signals are recorded. Given sufficient time, the outcome of a run may be either that the virus infects all of the parenchymal cells resulting in the death of the tissue (a viral "win") or the elimination of the virus and all virally infected cells with regeneration of healthy cells and restoration of the tissue to equilibrium conditions (an immune system "win"). Consistent with the theoretical properties of a complex system, our experiments have found initial conditions that always produce the same win/loss results, but the profiles of cell proliferation and signal production that occur are unique for every run of the simulation. Other initial conditions have been found that produce varying win/loss ratios. We plan to be able to use our simulation to explore formative patterns of agent behavior that develop within a complex adaptive system, to evaluate how information is used for decision making as responses evolve, and to develop methods of generating and evalulating simulator data that can be used to identify the strengths and weaknesses of clinical and experimental tools that are currently in use.

3. Careless about needs or rights of individuals Individuals are insignificant and expendable No recognized individuality No independent thought or creativity No leaders, no rule books, no blueprints Network of autonomous peers, each influencing the others Each are thoughtless slaves to environmental cues Leukocytes are more like ants than humans Even human leukocytes are inhuman Colony rules prevail; Human social rules are irrelevant,

4. Leukocytes always operate “en masse” as leukocyte swarms. Colony Rules for Leukocytes There are unlimited numbers of several different types of leukocytes. Each individual leukocyte operates as an independent entity. Each leukocyte has a finite set of genetically defined behavior patterns. All leukocytes can monitor and integrate many environmental signals, including those made by other leukocytes (pheromone equivalent). Each leukocyte responds predictably to defined patterns of environmental signals. Immune responses are leukocyte swarm functions. Leukocyte swarm functions are unpredictable (due to changing local conditions).

5. Agents • Parenchymal Cellsimpart tissue function • Dendritic Cellstissue surveillance, antigen presentation • Macrophagesscavenging, antigen presentation • T Cellslymphocytes, cell mediated immunity • Natural Killer Cellskill stressed cells • B Cellslymphocytes, humoral immunity (Antibodies) • Portalsblood vessels, lymphatic ducts

6. Agents and the Signals That They Produce Parenchymal CellsPK1 (Heat Shock Protein, a stress signal), Virus DC1 (Pro-inflammatory) Dendritic Cells DC2 (Anti-inflammatory) Natural Killer CellsCK1 (IFN-g),Apoptotic Bodies MO1 (Pro-inflammatory) MK1 (IL-12) Macrophages MO2 (Anti-inflammatory) MK2 (IL-10) Ab1 (cytotoxic) B Cells Ab2 (targeting) T1 (Pro-inflammatory) CK1 (IFN-g) T Cells T2 (Anti-inflammatory) CK2 (TGF-b) Necrosis factors, complement

7. Design: Three zones of activity Zone 1: Tissue Equivalent Blood Equivalent Zone 2: LN Equivalent Zone 1: Tissue Equivalent Zone 3: Blood Equivalent

8. Initial Studies: Characterization How do major changes in conditions affect the outcome? Given the same initial conditions, how reproducible is the outcome? Does the immune simulator behave as a complex system?

9. What happens when particular agents or signals are excluded? Time to Eliminate Infected Cells Immune Win Control 100% 117.9 +/- 17.4 No Dendritic Cells 0% NA No Antibodies 0% NA No Macrophages 0% NA No T Cells 0% NA No T1 Cells 30% 159 +/- 16.1 (p < .001) No T2 Cells 100% 121.8 +/- 15.4 (NS) No NK Cells 100% 171.8 +/- 21.6 (p < .0005)

10. What happens when the initial number of Dendritic Cells is varied?

11. T0 T0 Win Lose T1 T2 T2 T1 T1 T2 T0 T0 Lose Win T2 T1 Highly Variable Behavior of T Cell Populations Four consecutive runs with the same parameter settings Multiple runs with the same parameter settings do not necessarily yield the same outcome Even with locked parameter settings, the pattern of agent activity always differs for each run

12. Does the addition of memory cells enhance the simulated immune response? No Memory Memory (10 cells) Memory (50 cells) Time to Eliminate 117.9 +/- 17.4 107.6 +/-13.1 100.6 +/- 6.5 Infected Cells (p < .025) (p < .0005) Time to Appearance 32.2 +/- 9.9 23.9 +/- 8.7 18.7 +/- 6.9 of Antibody in Tissue (p < .01) (p < .0005) Time to Appearance 23.8 +/- 5.2 19.6 +/- 0.7 21.5 +/- 3.6 of T1 Cells in Tissue (p < .0005) (p < .05)

13. Observations: Initial conditions exist that always produce the same (win/loss) results. Every run of the simulator has a unique profile of cell proliferation and signal production. Initial conditions exist that produce varying win/loss ratios. The immune simulator remains cohesive when faced with change, ie. it contiues to function. The enhancement of the simulated immune response by “memory cells“ demonstrates the capability to learn. The activity of the immune simulator adapts to major changes in agent profiles. Conclusion: The immune simulator behaves as a complex, adaptive system.