IMMUNITY TO R. EQUI
Among equids, R. equi infections occur almost exclusively among foaIs; infected adult horses generally have an underlying immunodeficiency, and
human cases of R. equi are most commonly reported among persons infected with HIV or with other forms of immunosuppression such as that induced
by drugs in transplant recipients and those receiving chemotherapy. These findings suggest that there is a maturational defect
in the immune system of foals. The nature of this deficiency remains elusive. It is clear that immunity to R. equi in foals is complex and involves innate and adaptive immune responses.
Innate immune responses are critical for protection against infections in neonates because there is a lag until an adaptive
immune response will be developed. Epidemiological and clinical evidence exists that foals become infected early in life with
R. equi,1,2 such that innate immunity would be expected to be important for protection. Foal neutrophils can kill R. equi
3-5 and play a key role in protecting mice against experimental infection with R. equi.6 Neutrophils of newborn foals have been documented to have some functional deficits relative to those of older foals, including
cytokine expression and bactericidal capacity in some foals.6,7 Neutrophil concentrations were significantly lower in foals that subsequently developed R. equi pneumonia than in foals matched for age and farm.8 Although neutrophils are key effectors of innate immunity, other cells also play a role. Macrophages and peripheral blood
mononuclear cells express cytokines following stimulation with R. equi.9-11 The role of epithelial cells in innate immunity to R. equi is beginning to be studied. In brief, innate immune responses result both in direct killing of R. equi and likely contribute to directing the adaptive immune response towards a T helper lymphocyte type 1 (Th1) response.
In terms of adaptive immunity, evidence of a role for both humoral and cell-mediated immune responses exists. Prophylactic
transfusion of foals with hyper-immune plasma is partially effective in reducing disease incidence. Purified antibodies against
the virulence-associated proteins A and C (VapA and VapC) protected foals against experimental challenge.12 Opsonization with antibodies from commercial hyperimmune plasma enhanced the oxidative burst of neutrophils and macrophages,
and increased TNF-α production by macrophages.13 Moreover, opsonization results in killing of R. equi by equine neutrophils, and enhances phagocytosis and killing by macrophages.3,4,9 There is conflicting evidence regarding the efficacy of vaccination of mares to protect foals,14-17 with some data to suggest the potential for maternal transfer of immunity to foals.
Rhodococcus equi is a facultative intracellular pathogen. Evidence exists that cell-mediated immunity is critical for protecting against
intracellular infections. Roles for both CD4+ and CD8+ T lymphocytes have been described in mice and horses, with CD4+ playing
a dominant role in mice.18-22 In horses, CD8+ T cells recognize and kill R. equi-infected macrophages in an MHC class-I unrestricted manner.23,24 These data suggest that lipid moieties in the cell wall recognized by CD1 may be targets for protective immune responses.
Indeed, macrophages pulsed with R. equi lipids are lysed by CD8+ T cells in an MHC I-unrestricted manner.25 There is some controversy regarding the extent to which adaptive immune responses are impaired in foals, but most results
suggest that deficiencies in adaptive immune responses exist in foals.
The presumed route of infection for R. equi pneumonia is by inhalation. Thus, respiratory mucosal immunity is likely to play a role in protection. Mucosal immune responses
at a surface may be disseminated to other surfaces. The fact that oral vaccination of young foals with R. equi can protect against subsequent intrabronchial challenge suggests a critical role for mucosal immunity.26,27
In summary, immunity to R. equi is complex and multi-faceted. This complexity coupled with limitations of neonatal immune responses represent important
challenges for developing an effective vaccine against R. equi.