Immune responses require the tight control of dose, location, strength and duration through genetic, epigenetic or biochemical regulation. Of these, the generation of alternatively-spliced constructs increases transcriptional and proteomic diversity in post-transcriptional modification, localization and functional domain integrity. Specifically, this thesis explores how splice variation engenders profound differences in the biological functions of interleukin-22 (IL-22) binding protein (IL-22BP) and interferon lambda 4 (IFNλ4), which are both central components of distinct cytokine pathways in mucosal immunity and inflammation. IL-22BP is a soluble receptor for IL-22 that is expressed as three isoforms in humans, though the physiological relevance of the three human isoforms has remained a mystery due to the absence of this variation in mice. We present novel findings that IL-22BPi1 is inactive due to intracellular retention by its unique exon, while IL-22BPi3 is also an antagonist but with differential activity from IL-22BPi2. Importantly, while IL-22BPi3 has widespread expression in steady-state homeostatic conditions, IL-22BPi2 is the only isoform induced by inflammatory TLR2/retinoic acid stimulation, highlighting important spatiotemporal control of the two isoforms that exploit their differential activities. IFNλ4 presents a different mystery in which the protein-coding variant is genetically associated with poorer clearance, but the mechanism for this association remains unclear. We investigated several non-canonical functions proposed by the field, including intrinsic differences in activity of the three protein isoforms and their interference with antiviral activites of other type I or III interferons. Establishing an overexpression system and purifying recombinant proteins, we found that only the full-length isoform is active and exhibits similar effects to canonical type III IFN IFNλ3, without any blockade of other IFN signaling. Simultaneously, functional IFNλ4 expression is suppressed in hepatocytes and dendritic cells through preferential splicing to increase intron retention and expression of inactive isoforms. Therefore, alternative splicing in IFNλ4 is an important mechanism to control IFNλ4 bioactivity. The divergent manners in which alternative splice forms impact the activity of both IL-22BP and IFNλ4 highlight the important contributions of this process to cytokine biology and bigger implications that escape detection by genomic analyses.
Alternative splicing takes charge of interleukin-22 and interferon lambda 4 signaling
