Which bacteria pose a high risk of infection in patients with T cell defects?

Bacterial Pathogens That Pose High Risk in Patients with T Cell Defects

Patients with T cell defects are at particularly high risk for infections with intracellular pathogens including mycobacteria, Listeria monocytogenes, and Salmonella species, as well as fungal and protozoal organisms that require intact cell-mediated immunity for control. 1

Common Bacterial Pathogens in T Cell Deficiency

Intracellular Bacterial Pathogens

• Mycobacteria - Both typical (M. tuberculosis) and atypical mycobacteria pose significant risk 2
• Listeria monocytogenes - Requires T cell-mediated immunity for clearance 3
• Salmonella species - Particularly problematic in patients with Mendelian susceptibility to mycobacterial disease 1
• Nocardia species - Often causes pulmonary or disseminated disease

Other High-Risk Bacterial Pathogens

• Pneumocystis jirovecii - Though technically a fungus, requires prophylaxis in severe T cell defects 1
• Legionella pneumophila - Causes severe pneumonia in immunocompromised hosts

Severity Based on Type of T Cell Defect

Severe T Cell Deficiencies (e.g., SCID, Complete DiGeorge)

• Highest risk for all pathogens listed above
• Require comprehensive antimicrobial prophylaxis
• Cannot receive live bacterial vaccines (BCG, Salmonella typhi Ty21a) 1
• Pneumocystis prophylaxis is mandatory 1

Partial T Cell Deficiencies (e.g., Partial DiGeorge, Wiskott-Aldrich)

• Intermediate risk for intracellular pathogens
• Risk correlates with CD4+ T cell count:
  • <500 cells/mm³: High risk (adults)
  • <1000 cells/mm³: High risk (children 1-6 years)
  • <1500 cells/mm³: High risk (infants <1 year) 1

Prophylaxis Recommendations

Antibacterial Prophylaxis Options

• Trimethoprim/sulfamethoxazole: 5 mg/kg daily or twice daily (children); 160 mg daily or twice daily (adults) 1
• Azithromycin: 10 mg/kg weekly or 5 mg/kg every other day (children); 500 mg weekly or 250 mg every other day (adults) 1

Special Considerations

• For patients with severe T cell defects awaiting immune reconstitution (e.g., HSCT), aggressive prophylaxis against multiple pathogens is required 1
• Patients with Mendelian susceptibility to mycobacterial disease may benefit from IFN-γ therapy 1

Clinical Pearls and Pitfalls

Important Clinical Pearls

• The presence of unusual or severe viral, fungal, or protozoal infections should prompt investigation for T cell deficiency 4
• Recurrent or severe infections with common organisms, infections with unusual organisms, and failure to thrive are warning signs of immune dysfunction 5

Common Pitfalls

• Pitfall #1: Assuming normal bacterial flora are safe in T cell deficient patients - even commensal bacteria can cause invasive disease
• Pitfall #2: Delaying prophylaxis while awaiting definitive diagnosis - empiric coverage should be started promptly in suspected cases
• Pitfall #3: Administering live bacterial vaccines (BCG, oral typhoid) to patients with T cell defects, which can cause disseminated infection 1

Monitoring and Follow-up

• Regular surveillance for breakthrough infections
• Periodic assessment of T cell counts and function when possible
• Prompt and aggressive treatment of any suspected infection, often with broader coverage than would be used in immunocompetent hosts 1

Remember that the specific risk profile depends on the exact nature and severity of the T cell defect, with more profound deficiencies carrying higher risk for opportunistic and unusual pathogens.