Why spawning salmon return to their natal stream: the immunological imprinting hypothesis

Variations in Rainbow Trout Immune Responses against A. salmonicida: Evidence of an Internal Seasonal Clock in Oncorhynchus mykiss

In poikilothermic vertebrates, seasonality influences different immunological parameters such as leukocyte numbers, phagocytic activity, and antibody titers. This phenomenon has been described in different teleost species, with immunological parameters peaking during warmer months and decreased levels during winter. In this study, the cellular immune responses of rainbow trout (Oncorhynchus mykiss) kept under constant photoperiod and water temperature against intraperitoneally injected Aeromonas salmonicida during the summer and winter were investigated. The kinetics of different leukocyte subpopulations from peritoneal cavity, spleen, and head kidney in response to the bacteria was measured by flow cytometry. Furthermore, the kinetics of induced A. salmonicida-specific antibodies was evaluated by ELISA. Despite maintaining the photoperiod and water temperature as constant, different cell baselines were detected in all organs analyzed. During the winter months, B- and T-cell responses were decreased, contrary to what was observed during summer months. However, the specific antibody titers were similar between the two seasons. Natural antibodies, however, were greatly increased 12 h post-injection only during the wintertime. Altogether, our results suggest a bias toward innate immune responses and potential lymphoid immunosuppression in the wintertime in trout. These seasonal differences, despite photoperiod and water temperature being kept constant, suggest an internal inter-seasonal or circannual clock controlling the immune system and physiology of this teleost fish.

The intersection of stress, sex and immunity in fishes

While sexual dimorphism in immune responses has been documented in other vertebrates, evidence for a similar phenomenon in fish is lacking. Here, we review the relationship between immunity, stress, spawning, and sex hormones in fish to gain a better understanding of sex-based differences in fish immune responses and its consequences for aquaculture. It is well known that there is a strong link between the stress response and immune function in fish. In addition, research to date has demonstrated that sexual dimorphism in the stress response exists in many species; yet, the relationship between the sexual dimorphic stress responses and immune function has rarely been explored together. Aside from stress, spawning is also known to trigger changes in fish immune responses. Estrogens and androgens have been shown to modulate the immune system which could account for differences between the two sexes of fish when spawning; however, evidence regarding the sexual dimorphism of these changes varies between fishes and is likely related to the spawning strategy employed by a given species. Sex hormones are also used in aquaculture practices to produce monosex populations, and exposure to these hormones early in development has been shown to impact the development of immune organs in several fishes. While female fish are generally thought to be more robust than males, aquaculture practices should also consider the role that maternal stress has on the immune function of the offspring and what role this plays in compromising the immune response of farmed fish.

Transient increase in abundance of B lineage but not myeloid-lineage cells in anterior kidney of sockeye salmon during return migration to the natal grounds

As anadromous fish, sockeye salmon undergo complex endocrine changes when they return to their natal grounds to spawn. This is correlated with major immunological changes that will affect their response to pathogens. In spite of these challenges, salmon need to maintain sufficiently robust immunity to survive until spawning is complete, but the nature of immune adaptations during the spawning stage remains poorly understood. Our central question is to determine if sockeye salmon stimulate their immune system during the return migration and if so, whether this is a protective response. To begin answering this question, here we characterized the nature and timing of potential changes in anterior kidney immune fingerprints between salmon collected from seven different sites along the Kenai river, including the mouth of the river and two spawning sites. Our results revealed significant changes in abundance of B lineage, but not myeloid lineage cells during the spawning journey. This included early, transient and significant increases in abundance of both IgM+ and IgT+ B cells soon after fish entered the river, followed by a transient, significant increase in abundance of IgM++ secreting cells in fish caught mid-river, and ending with a return to base levels of both cell populations in fish caught at spawning sites. Further, males appeared to have higher immune activation than females, as reflected by higher abundance of IgM++ secreting cells, higher spleen index, and higher titers of serum IgM. Although roles for these newly generated IgM++ secreting cells remain unclear at this time, the data complement our previous work which supported roles for long-lived plasma cells to protect returning salmon from pathogens at their natal grounds. We conclude that sockeye salmon are capable of inducing B cell responses during their spawning journey, with males having stronger responses compared to females. B cell activation during the return journey may provide returning adults with additional protection against pathogens not encountered as juveniles.

Cumulative Effects of Thermal and Fisheries Stressors Reveal Sex-Specific Effects on Infection Development and Early Mortality of Adult Coho Salmon (Oncorhynchus kisutch)

Multiple stressors are commonly encountered by wild animals, but their cumulative effects are poorly understood, especially regarding infection development. We conducted a holding study with repeated gill and blood sampling to characterize the effects of cumulative stressors on infection development in adult coho salmon. Treatments included chronic thermal stress (15°C vs. 10°C) and acute gill net entanglement with an air exposure (simulating fisheries bycatch release). The potential loadings of 35 infectious agents and the expression of 17 host immune genes were quantified using high-throughput quantitative polymerase chain reaction, while host physiology was characterized with chemical analysis of blood. Temporal increases in infectious agent richness and loads were concurrent with decreased expression of immune genes in fish sampled in the river. In the laboratory, mortality was minimal in cool water regardless of fishery treatment (<15%). Elevated water temperature under laboratory conditions increased mortality of males and females (8% and 28% mortality, respectively, delayed by >1 wk) and enhanced mortality associated with handling and biopsy (∼40% both sexes). Experimental gillnetting at high temperature further enhanced female mortality (73%). Fish held at high temperature demonstrated heavier infectious agent loads, osmoregulatory impairment, suppressed female maturation, and upregulation of inflammatory and extracellular immune genes. At high temperature, heavy Parvicapsula minibicornis loads were associated with premature mortality. Females exhibited physiological impairment from both stressors after 1 wk, and infection burdens correlated poorly with immune gene regulation compared with males. Cumulative effects of multiple stressors on female mortality are likely a function of physiological impairment and enhanced infections at high temperature.

A cold-blooded view of adaptive immunity

The adaptive immune system arose 500 million years ago in ectothermic (cold-blooded) vertebrates. Classically, the adaptive immune system has been defined by the presence of lymphocytes expressing recombination-activating gene (RAG)-dependent antigen receptors and the MHC. These features are found in all jawed vertebrates, including cartilaginous and bony fish, amphibians and reptiles and are most likely also found in the oldest class of jawed vertebrates, the extinct placoderms. However, with the discovery of an adaptive immune system in jawless fish based on an entirely different set of antigen receptors - the variable lymphocyte receptors - the divergence of T and B cells, and perhaps innate-like lymphocytes, goes back to the origin of all vertebrates. This Review explores how recent developments in comparative immunology have furthered our understanding of the origins and function of the adaptive immune system.

The humoral immune system of anadromous fish

The immune system of anadromous fish is extremely complex, a direct consequence of their diadromous nature. Hormone levels fluctuate widely throughout their life cycle, as fish move between fresh and salt water. This poses major challenges to the physiology of anadromous fish, including adaptation to very different saline environments, distinct pathogen fingerprints, and different environmental stressors. Elevated cortisol and sex hormone levels inhibit B lymphopoiesis and IgM⁺ antibody responses, while catecholamines, growth hormones and thyroid hormones are generally stimulatory and enhance the humoral immune response. Immunological memory in the form of long-lived plasma cells likely plays important roles in health and survival during the life cycle of anadromous fishes. This review discusses some of the complex immune-endocrine pathways in anadromous fish, focusing on essential roles for B lineage cells in the successful completion of their life cycle. A discussion is included on potential differences in immuno-competence between wild and hatchery-raised fish.

Sockeye salmon immunoglobulin VH usage and pathogen loads differ between spawning sites

The Immunological Imprinting Hypothesis proposes that juvenile anadromous fish respond to the pathogen fingerprint specific to their natal site by producing protective long lived plasma cells (LLPCs) that constitutively produce antibodies against those pathogens. Hence, fish returning to their natal streams have immunological protection from pathogens at that specific location. Here, we tested the hypothesis through analysis of antibody composition and usage in sockeye salmon populations in Alaska. Spleen and anterior kidney were sampled from salmon from six sites, and relative usage levels of six different Immunoglobulin V_H gene families determined using RT-qPCR. Additionally, prevalence and pathogen loads were measured in each fish for Renibacterium salmoninarum, Flavobacterium psychrophilum, and Infectious Hematopoietic Necrosis Virus. Results revealed differences in V_H usage, pathogen loads, and infection rates between spawning sites, while probability of infection was dependent on location for each pathogen analyzed. Further, several negative correlations between specific V_H usage patterns and pathogen loads were uncovered. Greater understanding of site-dependent V_H usage in spawning fish potentially suggests a method of natural immunization against common fish pathogens and thus protection of both farmed and wild populations.

Innate and adaptive immune responses in migrating spring-run adult chinook salmon, Oncorhynchus tshawytscha

Adult Chinook salmon (Oncorhynchus tshawytscha) migrate from salt water to freshwater streams to spawn. Immune responses in migrating adult salmon are thought to diminish in the run up to spawning, though the exact mechanisms for diminished immune responses remain unknown. Here we examine both adaptive and innate immune responses as well as pathogen burdens in migrating adult Chinook salmon in the Upper Willamette River basin. Messenger RNA transcripts encoding antibody heavy chain molecules slightly diminish as a function of time, but are still present even after fish have successfully spawned. In contrast, the innate anti-bacterial effector proteins present in fish plasma rapidly decrease as spawning approaches. Fish also were examined for the presence and severity of eight different pathogens in different organs. While pathogen burden tended to increase during the migration, no specific pathogen signature was associated with diminished immune responses. Transcript levels of the immunosuppressive cytokines IL-10 and TGF beta were measured and did not change during the migration. These results suggest that loss of immune functions in adult migrating salmon are not due to pathogen infection or cytokine-mediated immune suppression, but is rather part of the life history of Chinook salmon likely induced by diminished energy reserves or hormonal changes which accompany spawning.

Sockeye salmon retain immunoglobulin-secreting plasma cells throughout their spawning journey and post-spawning

Antibody-producing plasma cells are a major source of protective immunity in vertebrates, including salmon. During the spawning phase, salmon undergo drastic, hormonally driven changes in their physiology, including elevated levels of cortisol, which are known to suppress the immune system. As adult fish need to survive their long journey to the spawning grounds, we hypothesized that humoral immunity, in the form of IgM-secreting plasma cells, remains functional until post-spawning. This was investigated by measuring changes in membrane and secreted immunoglobulin heavy chain mu and Pax5 transcripts in spleen and kidney from migrating sockeye salmon, using real-time qPCR. As an additional measurement, the abundance of developing B, mature B, and plasma cells was determined in spawning fish, using flow cytometry. Immune tissue samples were collected from fish from the Kenai River drainage and Main Bay, Prince William Sound. Our results reveal that spawning fish express high levels of secreted heavy chain mu transcripts in their spleen and anterior kidney throughout the spawning journey. Furthermore, we show that IgM-secreting PCs (HCmu++/Pax5-) remain abundant in anterior kidney and spleen of post-spawning sockeye salmon, with a concomitant loss in developing B cells (HCmu-/Pax5+). This suggests that successful spawners retain their PCs throughout the spawning journey and post-spawning.