Microscopic gossip: quorum sensing in the adaptive immune system
Summary
When an infection occurs in the human body, the body responds by sending many different types of immune cells to the infection. Some of these cells can stop the infection, others can create memory cells that can attack quickly if the infection happens again. A family of cells that is very important in this process is T cells. There are many different types of T cells. For example, T effector cells come in different kinds depending on the type of infection they respond to and they coordinate other parts of the immune system. T regulatory cells make sure that the body’s response does not become so big that it hurts the healthy tissue. T memory cells take care of the response to a secondary infection. For an efficient response all of these types of cells should be present at appropriate proportions. This can be regulated through limiting the amount of nutrients that is available for each cell type, or through naturally occurring death of old cells. Another mechanism that has been studied is quorum sensing. Quorum sensing was first described in bacteria and refers to a shared response of a whole population of bacteria to the density of the population. That is, at low bacterial densities, the bacteria show individual behaviour. However, when the density becomes high enough, they can change their behaviour into a group-level behaviour such as producing light. The bacteria can sense their density by secreting and taking up a substances called autoinducers. While this phenomenon is known mostly from microbial organisms, several studies suggest that this process could also contribute to the functioning of the human immune system.
This paper evaluates four mathematical models found in literature to see how quorum sensing contributes to immune responses and how accurately T cells can respond to their local density. Two of the models explain how, using the autoinducer IL-2, T cells can decide when they should start or stop an immune response. For example, if too little T cells respond to a substance in the body, the body decides that substances must belong to the body itself and therefore does not count as an infection. This means no immune response will be started. When enough T cells respond, however, an immune response will be started. The other two models elaborate on the influence of quorum sensing on the differentiation of T cells. They try to explain how the T cell density can determine which type of T cell other cells change into. These models suggest that the T cell density can influence cells to become a specific type of T effector cell or even a T memory cell. All of the behaviours that are modelled imply that responses resembling quorum sensing can occur in the immune system and that it is worthwhile to further study these processes.