A 28-year-old woman from Mumbai with known HIV infection (diagnosed 2 years ago, on antiretroviral therapy) presents with acute fever, headache, and photophobia. Lumbar puncture shows clear cerebrospinal fluid (CSF) with elevated protein (120 mg/dL), normal glucose, and negative bacterial culture. CD4+ count is 180 cells/µL. India ink stain of CSF is positive. Which structural feature of the causative organism is most relevant to its pathogenesis in this immunocompromised host?

Explanation

## Cryptococcal Meningitis: Pathogenesis and Structural Virulence Factors ### Clinical Context The patient presents with meningitis in the setting of CD4+ <200 cells/µL. India ink stain positive CSF is pathognomonic for *Cryptococcus neoformans*. This is an AIDS-defining illness that occurs when cell-mediated immunity is severely compromised. **Key Point:** *Cryptococcus neoformans* is an encapsulated yeast that causes meningitis in severely immunocompromised patients (CD4 <100 cells/µL). The polysaccharide capsule is the primary virulence factor. ### The Polysaccharide Capsule: Central to Pathogenesis 1. **Inhibition of phagocytosis**: The capsule is poorly immunogenic and does not trigger effective opsonization by antibodies or complement. Phagocytes cannot engulf the organism efficiently. 2. **Complement evasion**: The capsule prevents deposition of C3b and formation of the membrane attack complex (MAC), allowing the organism to survive in the CSF. 3. **Antiphagocytic effect**: The large, gelatinous capsule (up to 200 µm) physically prevents neutrophil and macrophage engulfment. 4. **Immunosuppression**: Shed capsular polysaccharides can act as immunosuppressive agents, further impairing the host response. **High-Yield:** India ink stain demonstrates the capsule as a clear halo around the yeast cell — the negative space is the hallmark of cryptococcal identification. ### Why CD4+ <100 cells/µL? Cryptococcal meningitis is a CD4 <100 cells/µL opportunistic infection. T cell immunity (especially Th1 response) is essential for controlling *Cryptococcus*. Without adequate CD4+ T cells, the organism disseminates from lungs to CNS. **Clinical Pearl:** Cryptococcal antigen (CrAg) testing in serum or CSF is highly sensitive and specific; CSF CrAg positivity confirms the diagnosis. Lumbar puncture findings (elevated protein, normal/low glucose, negative bacterial culture) are typical. ### Comparison: Virulence Factors of Common CNS Pathogens in HIV | Organism | Virulence Factor | Mechanism | CD4 Threshold | | --- | --- | --- | --- | | *Cryptococcus neoformans* | Polysaccharide capsule | Antiphagocytic, complement evasion | <100 cells/µL | | *Toxoplasma gondii* | Dense granules, rhoptries | Intracellular survival, immune evasion | <100 cells/µL | | *Mycobacterium tuberculosis* | Mycolic acid cell wall | Intracellular survival | <50 cells/µL (disseminated) | | *Cytomegalovirus* | Viral immune evasion genes | MHC downregulation, apoptosis inhibition | <50 cells/µL | [cite:Harrison 21e Ch 197; Robbins 10e Ch 8]