Common Variable Immunodeficiency

M. Julia B. Felippe

■ Definition and Etiology Common variable immuno­deficiency (CVID) in the horse is a rare, late-onset, clinically relevant immunologic disorder of inadequate antibody produc­tion due to impaired B-cell differentiation in the bone marrow and consequent B-cell depletion from lymphoid tissues and blood.

■ Clinical Findings, Clinical Signs, and Differential Diagnoses Most patients are reported to have had a healthy life until the signs of disease manifest as late-onset recurrent bacterial infections and fevers, hypogammaglobulinemia or agammaglobulinemia, progressive B-cell lymphopenia or deple­tion, and poor response to protein vaccination (e.g., tetanus toxoid).¹ The most common clinical signs include recurrent pneumonia, sinusitis, meningitis and/or neurologic disorders, abnormal gait (ataxia), peritonitis, gingivitis, hepatitis, diarrhea, susceptibility to GI parasites, uveitis, conjunctivitis, and skin abscesses.^2-4 Meningitis may clinically present with marked depression, alternated by periods of normal appetite. Weight loss and muscle atrophy are common.

The humoral feature of this immunodeficiency predisposes to infections and diseases caused by encapsulated bacteria, which requires opsonization with both antibody and complement for effective phagocytosis and bacterial killing. Therefore common pathogens involved in infections include Staphylococcus spp., Streptococcus spp., Actinobacillus spp., and Klebsiella spp. Occasionally, fungal infections, such as Aspergillus spp., Bipolaris spp., P. jirovecii (formerly P. carinii), or R. equi infections, have been diagnosed in a small number of CVID-affected horses that were treated with glucocorticoids or presented with a concomitant CD4+ T-cell lymphopenia.^1,5 In addition, Borrelia burgdorferi meningitis, cranial neuritis, and radiculoneuritis have been described in horses with B-cell lymphopenia and hypogammaglobulinemia.^6-8

A differential diagnosis is lymphoma because some forms of this type of cancer may alter lymphocyte distribution and function, including B-cell lymphopenia and hypogammaglobu­linemia.

■ Clinical Pathology During bacterial infections, blood work may reveal leukocytosis with neutrophilia, often mature, although a left shift and toxic changes may be present. Intermit­tent lymphopenia (triggers the suspicion of an underlying immunodeficiency. Affected horses have been euthanized for financial reasons or due to poor quality of life either soon after the diagnosis or after a few years of antimi­crobial management. Survival for several years after diagnosis is extremely rare but has been documented in a patient with marked B-cell lymphopenia (a proinflam- matory state known as inflammaging.¹² A robust decrease in peripheral naive 1 cells has been a consistent finding in numerous studies in people and rodents.^22-25 Thymic involution is considered a likely major contributor to the failure to maintain a naive T-cell pool. In addition, it has been suggested that the loss of naive T cells may be the result of lifelong infection by viruses, especially cytomegalovirus or other chronic antigenic stimulants that promote a bias toward the frequency of memory cells.^26,27 The effect of age on naive T-cell populations has not been explored in the horse.

Serum cytokine profiles in the aged are typically those of 2829

a proinflammatory phenotype.²⁸,²⁹ Aged horses show similar cytokine profiles with increased gene expression of TNF-α, IL-6, IL-1β, IL-8, IFN-γ, IL-15, and IL-18.¹²,¹⁸ In addition, there are increased proinflammatory-to-anti-inflammatory cytokine ratios such as IL6/IL10 and TNF-α∕IL-10.²⁵ Serum cytokine concentration of TNF-α was found to be increased in aged horses in one study but not another.^18,30 Obesity also increases the frequency of mononuclear cells expressing inflammatory cytokines.³¹ Confounding factors that might affect serum TNF-α concentration in these and other studies include concurrent illness, inflammation, or season of sample collection.

One of the most consistent findings in immunosenescence is a decrease in the ability of lymphocytes to proliferate. Studies in rodents and people have shown impaired lymphocyte proliferation possibly involving a decrease in serum IL-2 concentration or IL-2 receptor expression.^23,32,33 However, a decrease in lymphocyte proliferation has also been reported independent of these two factors, suggesting that defects in intracellular signal transduction may also be a consequence of aging. Horses also experience an age-associated decrease in lymphocyte proliferation, which is not responsive to IL-2 supplementation and not associated with a change in lymphocyte 1118 35

IL-2 receptor expression.’’ This suggests that, similar to what has been observed in people, the age-associated defect in lymphocyte division in horses may be in intracellular signal­ing. In addition, progressive age-associated telomere shortening in equine peripheral blood mononuclear cells has been associated with decreased proliferation potential.

Several studies have assessed the ability of peripheral blood mononuclear cells (PBMCs) to respond to immune stimulation. Inflammatory response after stimulation of whole blood or PBMCs from healthy aged people results in a greater release of proinflammatory cytokines than that observed in nonaged

Ii i ∙ ∙ i a ∙ ι ∙ ∙ ∙ „2 37 39

adults, a condition known as trained innate immunity. ^2, Similar studies in aged horses have revealed an increase in TNF-α and IFN-γ production from PBMCs following stimulation.^23,3/

Although immunosenescence affects primarily adaptive immunity, changes in innate immunity occur as well.

One of the major health concerns regarding immunose­nescence in people is a failure of the geriatric population to develop an adequate protective titer following vaccination, particularly after influenza virus immunization. As a result, morbidity and mortality due to influenza infection is greatest in the very old. The response of aged horses to influenza vaccines has also been reported to be less robust than that of young or adult horses in several studies.^11,46 Muirhead and colleagues reported a decrease in Ig subisotypes IgGl (formerly IgG_a) and IgG_4/7 (formerly IgG_b) in aged horses, although single radial hemolysis titers suggested that both adults and aged horses had adequate response that was protective. Using a different vaccine, Horohov and colleagues¹¹ reported a tenfold decrease in resultant titers in aged ponies compared with nonaged adults, but it is not clear if adequate titers were achieved in either group. Another study using canarypox recombinant virus vector expressing the hemagglutinin antigen of influenza in older horses slightly enhanced antibody responses but did not affect cell-mediated immune responses.

Response to naive antigenic challenge has also been examined in horses older than 20 years of age. Contrary to what has been reported in primates, the magnitude of a primary antibody response did not decline with age.^46,48-50 When a rabies vaccine was administered to aged horses naive to rabies vaccine, the antibody titer after both the initial and the second immunization did not differ from that of nonaged adult horses.

Finally, given the importance of the gut microbiota in the function of the immune system, dysbiosis or microbiota remodeling with age may somehow affect immunosenescence and inflammaging, and the balance between proinflammatory and anti-inflammatory mediators.

Drug-Induced Immunosuppression body weight) of dexamethasone has shown to induce neutro­philia and lymphopenia, a decrease in the peripheral blood CD4+ T-cell population with a concomitant increase in the CD8+ T cells, and a consequent decrease of CD4/CD8 ratio; these effects are observed at 4 and 12 hours after the drug administration and gradually decrease over time in the first 48 hours.²⁵ Chronic treatment with glucocorticoids may lead to susceptibility to infection, hyperglycemia, hypokalemia (particularly isoflupredone), GI ulceration, delayed wound healing, osteoporosis, growth suppression, myopathy, and hypertension.^3,22 Parenteral, oral, and aerosolized corticoids reversibly suppress adrenocortical function, while the response to ACTH stimulation remains intact.^22,26

Aerosolized glucocorticoids have been used for the preven­tion and treatment of recurrent airway obstruction and inflammatory airway disease in the horse, and they were shown to decrease airway hypersensitivity, neutrophilic pulmonary inflammation, and the expression of IL-1β and IL-8 in isolated alveolar macrophages.^{18,19,22,23,27,28}

Aerosolized fluticasone proprionate may be detected in plasma for a minimum of 72 hours post administration.²⁹ The treatment of heaves-affected horses with inhaled fluticasone at therapeutic dosages for 11 months had no significant or detectable effect on innate and adaptive (both humoral and cell-mediated) immune function assays, including response to vaccinations.³⁰

Inhaled budesonide may be detected in the plasma of horses with recurrent airway obstruction at higher concentrations and at longer time points (beyond 96 hours) after administration in comparison with healthy horses.³¹

Azathioprine inhibits the de novo pathway of purine syn­thesis, which is essential for DNA and RNA production and consequently lymphocyte activation and proliferation.

Cyclosporine is a cyclic polypeptide that binds to the cytoplasmic receptor cyclophilin and subsequently to the catalytic domain of the cytoplasmic phosphatase calcineurin.³⁴ This binding inhibits dephosphorylation of transcription factors, most importantly the nuclear factor of activated T lymphocyte (NFAT), which prevents their translocation to the nucleus. Therefore the production of key elements of T-cell activation and proliferation (IL-2, IL-4, CD40L, TNF-α, IFN-γ, c-myc) is suppressed. Cyclosporine has been used in the horse for intravitreal treatment of recurrent uveitis and immune-mediated keratitis.³⁵ Potential adverse effects of cyclosporine include vasoconstriction, hypertension, nephrotoxicity, hepatotoxicity, and potential susceptibility to infectious organisms that require cell-mediated protection.

Rapamycin and tacrolimus are macrolide antibiotics that bind to the cytoplasmic FK binding protein (FKBP), and the resultant protein complex inhibits calcineurin. The effect includes cell-cycle arrest and decrease in the expression of inflammatory cytokines. Rapamycin ocular toxicity and distribu­tion have been studied for potential clinical use in the horse.³⁶

Cyclophosphamide (phosphoramide and acrolein) and chlorambucil (phenylacetic acid mustard) metabolizes alkylate DNA bases, resulting in mutagenic, cytotoxic, antiproliferative, and chemotherapeutic effects. Cyclophosphamide affects B cells and has been used in the treatment of autoantibody- mediated diseases. In horses, these drugs have been used for the treatment of lymphosarcoma.³⁷ Adverse effects associated with cyclophosphamide and chlorambucil therapies include anemia, leukopenia, alopecia, and secondary malignancies.

Vincristine is a vinca alkaloid that binds to tubulin in the mitotic spindle and prevents purine synthesis. Consequently it inhibits cell proliferation, resulting in antitumor and immu­nosuppressive effects. In horses, vincristine has been used for the treatment of immune-mediated thrombocytopenia, pemphigus foliaceus, and lymphosarcoma.³⁸ Mild neurologic adverse effects (proprioceptive deficits) and ileus may occur in the horse.

The development of species-specific monoclonal antibodies and the identification of appropriate antigenic targets have contributed to the advance of their use in medicine.³⁹ The great advantage of monoclonal antibodies is the specificity of response, which may minimize adverse effects. Examples of use include treatment of inflammation (targeting inflammatory cytokines) and cancer.⁴⁰