Like most other mammalian species, bats are homeotherms and maintain their body temperature at 35–39°C at considerable energy cost.14 Hibernation allows survival during winter months when food is unavailable and temperatures can drop below freezing. During hibernation, bats enter a state of torpor where their body temperature drops to near ambient temperatures with intermittent short periods of arousal. Torpor is accompanied by reductions in physical activity, metabolic rate, heart rate and respiratory rate, as well as a switch in metabolism from carbohydrate-based (glycolysis) to fat catabolism.14-16 Immune responses are temperature sensitive and metabolically costly17,18 and recent work suggests that aspects of immune function are also downregulated in bats during hibernation.18 Recent advances in microscopy tools that enable the tracking of immune cell movement in live mice19 have revealed the exquisite temperature sensitivity of immune cell motility and function; some cell types, such as lymphocytes, may be more sensitive than others (Mandl JN, unpublished results).20 Data are lacking on regulation of immune function in hibernating mammals and it is unclear whether they have adapted to retain specific immune functions even at low body temperatures. Traditional bat pathogens likely replicate less efficiently when the bat host is less metabolically active and have reduced transmission rates with reduced host mobility. If this is the case, the normal physiological immunosupression that occurs in the context of torpor may not necessarily render hibernating bats more susceptible to typical, co-evolved pathogens.
Very little is currently known about the immune system of bats, either during hibernation or their euthermic, active period.16 Although data from studies of immune function during hibernation in other mammals do exist, it is still unclear which aspects of immunity are altered during this downregulated state.20 As early as the 1960s, experimental infections of hibernating ground squirrels with Colorado tick fever virus showed that animals had protracted viremia and reduced antibody titers when they were infected just prior to induction of torpor.21 More recent studies showed that unlike euthermic animals, injections of LPS in hibernating ground squirrels did not result in fever.22 While this implies that innate immune cell function is altered in torpid animals, it remains unknown whether innate sensing pathways, cytokine production, cell recruitment or other aspects of innate immune cell function are affected. Furthermore, consistent with observed effects of temperature on the motility of immune cells, it is becoming clear that hibernation results in substantial changes in the trafficking behavior of cells of the adaptive immune system, T and B lymphocytes. Both in hibernating ground squirrels and Syrian hamsters, circulating blood lymphocyte counts are dramatically decreased compared with euthermic animals, with B and T cells being sequestered in secondary lymphoid organs at low body temperatures.23,24 Similarly, following the experimental induction of torpor using 5′AMP injected into mice,25 lymphocyte egress from secondary lymphoid organs is greatly reduced (Mandl JN, unpublished results). The retention of lymphocytes within secondary lymphoid organs during hibernation may be a means to ensure survival of this specialized population of cells. However, sequestration of lymphocytes would also prevent them from homing to sites of infection reducing both immune surveillance and limiting the generation of cellular immune responses that play an essential role in pathogen clearance. Further evidence that lymphocyte activation, function and/or homing are restricted during torpor is the successful maintenance of skin allografts transplanted during hibernation which are subsequently rejected when animals return to euthermia at the end of the hibernation season.26
Bats infected with WNS during hibernation not only show no gross evidence of pathology, but histopathology indicates that the initiation of inflammatory responses and/or the recruitment of immune cells to sites of G. destructans infection does not occur when animals are hibernating (Fig. 2A and B).11,13 The unique histology of G. destructans is diagnostic for bats with WNS and consists of dense aggregates of robust hyphae that form a defined interface with skin and erosion along the leading edge of contact.11 Yet, in spite of the invasive nature of G. destructans, neutrophils and macrophages are characteristically absent from sites of pathogen invasion in hibernating bats with WNS (Fig. 2B).11 Not only is the low body temperature of hibernating bats conducive to the replication of this novel emerging pathogen, but the absence of histologically visible inflammatory responses in the skin suggests that, at least during torpor, this fungus is not being limited by effective immune control.
Figure 2. (A) Little brown bat found February 8, 2009 frozen outside of the small opening of a copper mine. The transilluminated wing was photographed outstretched over a light box and shows no evidence of wing damage. (B) Periodic acid Schiff stained section of wing membrane from bat (A) shows characteristic dense aggregates of robust hyphae forming a defined interface with the skin, erosion along the broad zone of skin contact (arrows) and no visible inflammatory response. (C) One of nine little brown bats that were found on the ground and unable to fly between April 4 and May 7, 2012. This bat was collected April 4, taken into rehabilitation, ate and drank, but died within 18 h of arrival. The wing was photographed outstretched over a light box and visible damage can be seen with dark areas of contraction and loss of elasticity. (D) Periodic acid Schiff stained section of wing membrane from the bat in (C). Severe neutrophilic inflammation and edema (bracket) in response to fungal hyphae (arrow). (E) Different field from same slide as in (D) shows a thick layer of degenerating neutrophils (brackets) at the margins of a dense aggregate of fungal hyphae eroding epidermis (arrow). (F) Little Brown Bat in (C). Degenerating neutrophils (arrowheads) surround the dense aggregate of fungal hyphae (arrows).
During HIV infection, immunosuppression occurs as a consequence of the depletion of CD4 T cells, a critical immune cell population of the adaptive immune system. The HIV-induced loss of CD4 T cells results in host susceptibility to many opportunistic infections and the recovery of CD4 T cell function following treatment with antiretroviral therapy usually restores resistance to these microbial infections.31 However, within a few weeks after starting antiretroviral therapy, some AIDS patients undergo a rapid deterioration in symptoms rather than the expected clinical improvement. This paradoxical adverse event of antiretroviral therapy, IRIS, occurs most frequently in patients who are severely CD4 T cell deficient and harbor a microbial co-infection at the time of ART initiation.29,32,33 Interestingly, IRIS also tends to occur in individuals with the best and most rapid response to ART, as measured by the decrease in HIV viral loads, and with a sudden increase in CD4 T cell numbers. IRIS has been documented in patients with a diverse array of co-infections, and the manifestation of disease depends on the particular opportunistic pathogen and the site of infection. Fungal infections in particular are often associated with HIV-IRIS events, and meningeal infection with Cryptococcus neoformans in HIV positive individuals treated with antiretroviral therapy results in the most lethal form of IRIS.
The mechanisms of HIV-IRIS are not well understood,33 but the prevailing hypothesis suggests that when HIV viral replication is inhibited by ART, the ensuing recovery of CD4 T cells drives an over-exuberant destructive immune response against the underlying microbial co-infection with subsequent damage to infected tissue. Data from a recently developed model of experimentally induced mycobacterial IRIS support the idea that although CD4 T cells are normally required for control of mycobacterial infections, they can also mediate damaging responses during IRIS.34 It has been shown that both wild-type and T cell deficient mice are able to survive with disseminated M. avium infection for many months. However, when T cell deficient mice harboring an established M. avium infection are injected with purified CD4 T cells, the mice develop a severe inflammatory disease and die within 1 to 3 weeks after T cell transfer.34 This adoptive transfer of CD4 T cells does not lead to inflammatory disease in M. avium infected mice that have normal numbers of circulating T cells or in T cell deficient mice without M. avium infection. This basic observation illustrates the fundamental immunological phenomena of IRIS: once a microbial infection is established in an immunodeficient host, immune recovery can be more detrimental to the host than the opportunistic infection itself, at least in the short-term.
IRIS also occurs following recovery from other forms of immunosuppression.27-30 For example, tumor necrosis factor (TNF) blockade for the treatment of rheumatoid arthritis or Crohn disease can increase susceptibility to M. tuberculosis infection, but rapid removal of TNF blocking drugs after M. tuberculosis infection is established in these patients can further exacerbate the pathology.35 In some multiple sclerosis patients who are treated with the integrin blocking drug natalizumab to prevent lymphocyte migration into the CNS, a quiescent infection with JC polyoma virus may reactivate, leading to progressive multifocal leukoencephalopathy. Rapid removal of the integrin blocking drug can lead to severe worsening of CNS inflammation as the lymphocytes rush back into the brain in response to both the multiple sclerosis and JC polyoma virus replication.36,37 As a final example, patients with hematologic malignancies who receive chemotherapy to kill the malignant hematopoietic cells in the bone marrow can develop progressive aspergillosis as a result of the resulting neutropenia, but recovery of neutrophil numbers is sometimes associated with a worsening of pulmonary radiological findings and clinical symptoms despite signs of effective antifungal drug treatment.38
US Fish and Wildlife Service. North American bat death toll exceeds 5.5 million from white-nose syndrome.: http://www.fws.gov/WhiteNoseSyndrome/index.html, 2012.
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