Mechanisms of B Cell immunity in marsupials: A study of genes that regulate immunity

Marsupials are capable of producing antibodies in response to antigens, but questions remain about the timing, rate of class switching and magnitude of the B cell responses when compared to eutherian species such as humans and mice. While antibody production is at the centre of the mammalian humoral immune response, B cells also perform other functions in the immune system including antigen presentation, cytokine production and immune regulation. It is therefore essential to have a global understanding that encompasses the spectrum of B cell functions when attempting to assess B cell immunobiology and immunocompetence in marsupials. Little research to date has addressed marsupial B cell function beyond the scope of antibody production. The aim of this study was to identify if marsupials possess the molecular framework that may enable their B cells to perform these multifaceted functions. To achieve this, a survey of immune genes was performed on activated lymphocytes from the the tammar wallaby (Macropus eugenii), a model macropod marsupial. The genomic organisation and molecular expression of the central genes that are involved in B Cell antigen recognition and signal initiation (CD79a, CD79b, CD81, CD19), inter-cellular associations (CD40), intracellular signalling (Lyn, Syk, BLNK), and the cytokine (IL21) that regulates the B cell response were characterised in wallaby and some other distantly related marsupial species. To support these findings, protein expression studies were conducted for an endangered macropod species, the bridled nailtail wallaby (Onychogalea fraenata), using the data obtained from the closely related tammar wallaby as a template. Overall findings suggest that macropod marsupials not only express these key immune molecules, but also possess complex immune regulatory mechanisms such as alternative splicing; evident in the expression of their immune signalling molecules. It was also evident that post-translational modifications such as phosphorylation of tyrosine residues is an area of importance for future studies as key macropod signal transduction molecules possess more potential phosphorylation sites than their human counterparts. While low expressed sequence conservation and/or some differences in genomic organisation compared with eutherian homologues was found for some molecules in this study, the differences in antibody responses, previously attributed to immune deficiency, do not appear to be explained by a lack of complexity in the metatherian molecular framework that underpins most essential B cell functions.