Host Resistance to Bacterial Infection Varies Over Time, but Is Not Affected by a Previous Exposure to the Same Pathogen

The plasticity of immune memory in invertebrates

Whether specific immune protection after initial pathogen exposure (immune memory) occurs in invertebrates has long been uncertain. The absence of antibodies, B-cells and T-cells, and the short lifespans of invertebrates led to the hypothesis that immune memory does not occur in these organisms. However, research in the past two decades has supported the existence of immune memory in several invertebrate groups, including Ctenophora, Cnidaria, Nematoda, Mollusca and Arthropoda. Interestingly, some studies have demonstrated immune memory that is specific to the parasite strain. Nonetheless, other work does not provide support for immune memory in invertebrates or offers only partial support. Moreover, the expected biphasic immune response, a characteristic of adaptive immune memory in vertebrates, varies within and between invertebrate species. This variation may be attributed to the influence of biotic or abiotic factors, particularly parasites, on the outcome of immune memory. Despite its critical importance for survival, the role of phenotypic plasticity in immune memory has not been systematically examined in the past two decades. Additionally, the features of immune responses occurring in diverse environments have yet to be fully characterized.

Impact of Chronic Infection on Resistance and Tolerance to Secondary Infection in Drosophila melanogaster

Prior exposure to a pathogen can greatly influence the outcome of a secondary infection, and although invertebrates lack classically defined adaptive immunity, their immune response is still influenced by prior immune challenges. While the strength and specificity of such immune priming depends highly on the host organism and infecting microbe, chronic bacterial infection of the fruit fly Drosophila melanogaster with species isolated from wild-caught fruit flies provides broad nonspecific protection against a later secondary bacterial infection. To determine how chronic infection influences progression of secondary infection, we specifically tested how chronic infection with Serratia marcescens and Enterococcus faecalis impacted both resistance and tolerance to a secondary infection with an unrelated bacterium, Providencia rettgeri, by simultaneously tracking survival and bacterial load postinfection across a range of infectious doses. We found that these chronic infections increased both tolerance and resistance to P. rettgeri. Further investigation of S. marcescens chronic infection also revealed robust protection against the highly virulent Providencia sneebia, and that protection was dependent on the initial infectious dose for S. marcescens with protective doses corresponding with significantly increased diptericin expression. While the increased expression of this antimicrobial peptide gene likely explains the increased resistance, increased tolerance is likely due to other alterations in organismal physiology, such as increased negative regulation of immunity or tolerance of ER stress. These findings provide a foundation for future studies on how chronic infection influences tolerance to secondary infection.

Drosophila melanogaster as a model to study innate immune memory

Over the last decades, research regarding innate immune responses has gained increasing importance. A growing body of evidence supports the notion that the innate arm of the immune system could show memory traits. Such traits are thought to be conserved throughout evolution and provide a survival advantage. Several models are available to study these mechanisms. Among them, we find the fruit fly, Drosophila melanogaster. This non-mammalian model has been widely used for innate immune research since it naturally lacks an adaptive response. Here, we aim to review the latest advances in the study of the memory mechanisms of the innate immune response using this animal model.