Exploring the Innate Immune System: Using Complement-Mediated Cell Lysis in the Classroom

Abstract

When teaching biology, instructors often face the challenge of teaching various subdisciplines. One of the subdisciplines that often receives very little attention is the immune system. Part of the reason for this oversight is the perception that the immune system is complex and the appreciation that few typical high school or undergraduate college laboratories have the resources, e.g., tissue culture hoods and incubators, to examine the immune system extensively. When the immune system is addressed, the lessons often focus on the adaptive immune system and the charismatic T cells, B cells, and macrophage cells that comprise it. This article explains how complement-mediated bacterial cell lysis provides an opportunity to introduce students to the wonders of the innate immune system and specifically highlight the often-overlooked importance and properties of the complement system. The results of complement-mediated bacterial destruction are dramatic and, while the actual pathway is a rather complex cascade of protein activation, conceptually the process is both easily explained by instructors and understood by students. Additionally, the exercise presented here can be modified to examine a number of different variables of immune system function. An examination of complement-mediated cell lysis links the phenomena of microbiological growth, enzymatic and stoichiometric chemistry, protein-heat instability (lability), immune pattern recognition, protein activation, protein cascades, and the cooperation between the innate and adaptive immune systems. The lab presented here can act as a springboard into any of these topics. Background The Adaptive Immune Response The more familiar understanding of the immune response involves the activity of B and T cells. The capacity for certain B or T cells to be selected and expanded based on surface receptor/antigen engagement drives the concept of adaptive immunity. B and T cells have surface receptors that are created by a similar mechanism and which recognize single, unique foreign antigens. When B cell and T cell surface receptors encounter an appropriate foreign antigen, the individual cell (clone) undergoes rapid We term this phenomenon expansion. The clonal expansion of appropriate B and T cell occurs in the lymph nodes and other lymphoid tissues and is characterized by the familiar lymph node swelling observed during infection. This antigen-specific cell expansion and subsequent immune attack clears the body of pathogens. Antigen-specific memory B and T cells survive and provide immunologic memory. The rapid re-activation and expansion of these cells in future antigen encounters explains why certain pathogens do not infect us more than once and underlies the concept of vaccinations. The Innate Immune Response The adaptive immune system discussed above is fundamentally a reactive, second line defense against pathogens. The first line of defense against disease is the innate immune system. Innate immunity is composed of skin secretions, barriers to pathogen entry, phagocytic cells, and blood proteins that neutralize pathogens before or soon after they enter the body. One part of the innate immune response is the protein complement system discussed in this article. The complement system is a set of nine proteins found in the blood of all mammals and is thought to have existed in animals as far back as 300 million years ago. (Wood, 2006; Sunyer et al., 2005). It is responsible for the recognition and destruction of foreign pathogens including viruses, bacteria (von Lackum et al., 2005), fungi (Speth et al., 2004), and other single-celled organisms (Inal, 2004). The complement system acts through a protein activation cascade in which individual complement proteins bind to the targeted pathogen and subsequently recruit additional complement pathway proteins. Proteins involved in the complement process (referred to as C1 through C9) are inactive until they enter the cascade where they join the complex and become activated themselves. …