How
does the timing of humoral immune response to antigen exposure differ in
reptiles? Identical to mammals, reptiles’ lag period concerning the initiation
of the production of antigen-specific antibodies of mediocre affinity after
exposure is roughly one week. However, the optimum production rate of the
antigen-specific antibodies differs in that it occurs six to eight weeks after
exposure. In addition to the increased lag period, affinity maturation is not a
characteristic of ectotherms. Instead, non-specific or natural antibodies
(NAbs) of lower similarity to the antigen than antigen-specific antibodies are
produced in response to antigen stimulation. For you to understand the following
experiment, NAbs must be described in further detail. NAbs are most commonly
observed in the immunoglobulin M (IgM) isotope and are secreted as pentamers;
the pentameric structure of NAbs aids in targeting antigens to lymphatic
tissues to be destroyed and improves adaptive responses.
Zimmerman
et al. conducted an experiment using
red-eared slider turtles (a subspecies of Trachemys
scripta) to examine how the humoral immune system of reptiles is affected
by aging. Zimmerman and her colleagues studied the roles of NAb (similar to B-1
cells in mammals) and specific responses (resembling B-2 cells in mammals) by
analyzing outcomes of antibodies against LPS (lipopolysaccharide), a
T-independent antigen. A B cell can respond to LPS without direct contact with
a T cell. Why were red-eared sliders chosen to carry out this experiment? The main
argument as to why this species was chosen is that these turtles continue to
grow throughout their lifetime, which is about thirty to forty years; thus,
plastron length can serve as an alternative for age. Previous studies suggested
that there was an increase in total Ig levels with age in Trachemys scripta, and they are likely to be seasonally exposed to
LPS within their natural environment. Zimmerman et al. hypothesized that the
slow and less vigorous specific humoral responses in red-eared sliders
primarily utilizes a NAb-based response and therefore, would not exhibit a
downfall in humoral immune defenses with age. It was also predicted that based
on the type of predominantly used antibodies, the humoral response would change
across the active season as the turtles naturally encountered LPS in the
environment. As hypothesized, if they mostly use a NAb-based response, the
amount and the overall strength of binding between antigen-specific
antibodies (avidity), total Ig, and AbSC number and function are preserved. In contrast,
if the turtles primarily use an antigen-specific-based response, AbSCs would
show a reduced response to LPS stimulation, and avidity and the percentage of
lymphocytes would diminish with age; during the active season (early May, late
June, and late August) with high LPS exposure, LPS-Abs, total Ig, avidity,
number of AbSCs, the amount of antibody produced by each cell, and the
percentage of lymphocytes would increase.
Zimmerman and her colleagues found
that the quantity of LPS binding antibody concentrations (LPS-Abs)
significantly increased with plastron length (LEFT) and across the active season (RIGHT),
providing evidence that red-eared sliders utilize a predominantly NAb response
rather than an antigen-specific response seen in mammals. Please take note that the letters (a, b, a,b) exhibit significantly different sampling dates. There was no correlation sex on LPS-Abs, and the date by sex interaction was not significant.
Avidity did not correlate with plastron length (age), date, or sex; it ranged from 10-4-10-6, which is within
the range of avidity for NAbs (10-3-10-7)--another line of evidence suggesting that the red-eared sliders primarily use a NAb response. Total Ig varied significantly by date, ranging from 0-9,7mgml-1; a relationship between
total Ig and sex was found with males having significantly higher total Ig
levels than females. It is important to note that there was not a relationship
between total Ig and age (plastron length). The number of LPS-stimulated AbSCs and spot size did not vary with plastron length, date or sex. Total WBC (white blood cell) number greatly decreased with increasing age but did not vary with sex or date; although the WBC number changed with age, the distribution of leukocytes did not exhibit a change in response to age.
Trachemys scripta exhibited an increase in NAb, B-1-like immunity with increasing age; the specific, B-2-like response to LPS was not significantly impacted by age. How do the turtles maintain this immune responsiveness to LPS as they age? A previous study concluded that mice had a subset of B cells called CD5+B-1 cells that maintained their ability to respond to antigens with increasing age; Zimmerman et al. stated that there was a possibility that all of the red-eared slider turtles' B cells were CD5+B-1 cells, implying that antigen-specific immune responses did not exist within the turtles. A second possibility is that the B cells are activated through the Toll-like receptor pathway; these Toll-like receptors recognize structurally conserved molecules derived from microbes and activate immune cell responses once the microorganisms have surpassed physical barriers within the body.
Zimmerman et al.'s study on humoral immune responses in red-eared slider turtles (Trachemys scripta) supports the idea that NAbs are an important line of defense in reptiles; an increase in NAbs with age is viewed as a positive change in immune function of reptiles, but the shift to a predominantly B-1 response in humans is characterized as a negative change. The seasonal changes in immunity observed (LPS-Abs and total Ig) could be a result of increased pathogen pressure across the active season. What was found to be most intriguing was the distribution of the leukocyte population and the relationship between total Ig and sex. A previous study found that lymphocytes of red-eared sliders accounted for approximately 14% of the leukocyte population--a widely supported conclusion. In comparison, Zimmerman and her colleagues found that the lymphocytes of the turtles accounted for roughly 42% of the WBC population; similar data has been observed in most species of lizards...It is still uncertain as to why different results were obtained within the study. The only parameter that varied between sexes was total immunoglobulin (Ig); take note that previous studies did not find any sex differences in any immune response parameter measured, including total Ig. A recent study suggested that estrogens enhance T-dependent humoral responses but do not affect T-independent responses, such as LPS; this may provide an explanation as to why females did not exhibit higher total Ig levels than males. We are certain that humoral responses are maintained with age in red-eared sliders, but to fully understand why this phenomenon is observed, more research is needed.
author: Zimemrman et al.
jounal: The Journal of Experimental Biology
1 comment:
Are there implications for turtle conservation? Would you predict a difference between turtles and less long-lived reptiles?
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