Last week I attended a talk by Dr. Markus Mohrs where he introduced the idea of a dual-reporter mouse model that they had developed some years ago to measure cytokine production in vivo. It fascinated me so much that I decided to go through his research a bit and read about their exciting findings, especially because they used parasites such as Heligmosomoides polygyrus, Toxoplasma gondii and Schistosoma mansoni as their infection models.
Th2 immune response is highlighted by IL-4 production. But when looking in vivo, it is impossible to find the source of the IL-4 that is present in the biological fluids. So in their study published in Immunity in 2005, the authors used an intestinal infection by nematode H. polygyrus as their model. This worm resides solely in the intestine and induces a robust Th2 response. they engineered a GFP sequence preceded by an internal ribosome entry site (IRES) after the final exon of the IL-4 gene. In this way, when the IL-4 mRNA gets transcribed, the ribosome can bind separately to the IRES and translate GFP. Therefore, cells that are producing IL-4 mRNA could be identified. What they surprisingly found was that, CD4+ GFP+ T cells were not always producing IL-4 unless secondarily stimulated. So, they replaced the IL-4 allele on the other chromosome with a Human CD2 gene. Presence of huCD2 on cell surface would now be indicative of IL-4 production.
They therefore showed that CD4+ Tcells could be either ”IL-4 competent” or ”IL-4 producing”. They observe a very strong post-transcriptional control and selective secretion of IL-4 . Various types of cells start producing IL-4 mRNA but only select ones in certain areas actually produce the cytokine after receiving the proper secondary signal. This idea was tried on the key Th1 cytokine IFN-gamma in another study using influenza virus and Toxoplasma as infection models and similar results were found. Interestingly, the iconic pro-inflammatory cytokine IL-1beta also takes a similar route, but in this case, both control levels are post-translational: First signal induces pro-IL-1beta production and second signal induces its maturation via the inflammasome complex. These key cytokines have receptors on plenty of cells all through the body and their aberrant release can cause serious problems, no wonder multiple controls levels are set for their release. Studies of this kind should really alert us on interpretation of microarray or qRT-PCR data.
Mohrs K, Wakil AE, Killeen N, Locksley RM, & Mohrs M (2005). A two-step process for cytokine production revealed by IL-4 dual-reporter mice. Immunity, 23 (4), 419-29 PMID: 16226507