Classification & Structure
Taxonomy & Morphology
| Kingdom / Phylum | Fungi · Ascomycota · Pneumocystidomycetes; formerly classified as protozoan until 1988 rRNA analysis |
| Host specificity | Obligate human lung pathogen; cannot be cultured axenically; host-genus specific (P. jirovecii humans, P. carinii rats); not zoonotic |
| Trophic form | 1–4 µm uninucleate pleomorphic cell; attaches tightly to alveolar type I pneumocytes via fibronectin/vitronectin surface ligands; predominant form in infection |
| Ascus (cyst form) | 5–8 µm spherical thick-walled cyst with 8 intracystic bodies (ascospores); β-1,3-glucan-rich wall; detected by GMS (silver) and toluidine blue stains |
| No ergosterol | Cell membrane contains cholesterol (not ergosterol); intrinsically resistant to azoles (target Erg11) and amphotericin B (binds ergosterol); unique antifungal target requirements |
| Cell wall β-glucan | β-1,3-glucan in cyst wall; recognised by Dectin-1 on alveolar macrophages; shed into blood → serum β-D-glucan marker; not present in trophic forms |
| Msg / gpA antigenic variation | Major surface glycoprotein (Msg/gpA) ~100–120 kDa GPI-anchored; ~80 gene copies in subtelomeric arrays; single UCS (unique conserved sequence) expression site → cassette switching → antigenic variation; evades acquired immunity |
| Transmission | Airborne; primary infection in early childhood (seroconversion by age 2–4 in most populations); reactivation of latent lung reservoir OR re-exposure to exogenous source (both occur) |
| GMS stain appearance | Cysts: collapsed "crushed ping-pong ball" or crescent shape due to intracystic body release; trophic forms not visualised by GMS; DIF (direct immunofluorescence) most sensitive method |
Pathogenesis Mechanisms
- Alveolar attachment and type I pneumocyte injury Trophic forms attach tightly to alveolar type I pneumocyte surface via Msg/gpA binding to fibronectin, vitronectin, and surfactant proteins (SP-A, SP-D). Dense colonisation — hundreds of organisms per cell — causes physical distortion and functional impairment of gas exchange without direct cytolysis. Alveolar architecture is disrupted: intra-alveolar exudate fills with a characteristic "foamy" proteinaceous material (organisms + fibrin + desquamated cells). Type II pneumocyte hyperplasia is a compensatory response.
- Inflammatory lung injury — immune-mediated pathology Most severe lung damage is immune-mediated rather than direct fungal toxicity. Alveolar macrophages and neutrophils recruited to organisms shed β-glucan → Dectin-1 → IL-1β, TNF-α, IL-8 → neutrophilic alveolitis. CD4⁺ T-cell depletion impairs the regulatory anti-inflammatory response, paradoxically reducing initial inflammation — this is why PaO₂ can be near normal at presentation in AIDS patients. Upon ART/immune reconstitution, explosive T-cell-mediated inflammatory response can worsen oxygenation (PCP-IRIS).
- Msg antigenic variation — antibody evasion The ~80-copy MSG gene family encodes structurally diverse surface glycoproteins. Only one Msg variant is expressed at a time via the UCS promoter; expression cassette switches occur to change the displayed epitope. This mechanism mirrors VSG switching in trypanosomes. The result: host antibody response is consistently one step behind, preventing sterilising humoral immunity. B-cell-deficient and agammaglobulinaemic patients are also highly susceptible, indicating antibody plays a protective role.
- Surfactant disruption Dense trophic colonisation impairs surfactant function by sequestering SP-A and SP-D (innate opsonins that also maintain alveolar surface tension). Loss of surfactant → increased alveolar surface tension → atelectasis → VQ mismatch → hypoxia. Organisms also secrete protease activities that degrade surfactant proteins. Surfactant dysfunction explains why exertion dramatically worsens hypoxia in PCP (exercise-induced O₂ desaturation is a hallmark sign).
- Latency and subclinical carriage Seroepidemiological data indicate universal exposure by early childhood in most regions. In immunocompetent adults, low-level pulmonary colonisation occurs without disease. Immunosuppression — particularly CD4 depletion below 200/µL or corticosteroid doses ≥20 mg/day prednisone equivalent — allows unchecked proliferation. Asymptomatic carriers can transmit to susceptible contacts (hospital clusters documented).
- TMP-SMX mechanism and DHPS resistance TMP-SMX exploits a unique feature of Pj: its folate pathway enzymes have distinct binding pockets. SMX (sulfamethoxazole) inhibits dihydropteroate synthase (DHPS) → blocks para-aminobenzoate incorporation into dihydropteroate; TMP inhibits dihydrofolate reductase (DHFR). Combined blockade → no THF → no thymidylate synthesis → no DNA. Human DHFR is 10⁵× less sensitive to TMP than Pj DHFR. DHPS mutations at codons 55 (Pro→Thr/Ala) and 57 (Thr→Ala) arise under sulfonamide prophylaxis pressure; may reduce SMX efficacy but clinical resistance is not yet fully established.
Host Immune Response
Innate & Adaptive Defences
Disease Spectrum
| Syndrome | Population | Severity | Key Feature |
|---|---|---|---|
| PCP — mild | HIV CD4 200–350; mild immunosuppression | Mild | Exertional dyspnoea; dry cough; normal CXR or subtle GGO; PaO₂ >70 mmHg; ambulatory TMP-SMX treatment |
| PCP — moderate–severe (AIDS-defining) | HIV CD4 <200; first AIDS-defining illness in ~20–30% of cases in ART-naïve | Severe | PaO₂ <70 mmHg or A-a gradient >35 mmHg; bilateral GGO on HRCT ("ground glass" ± "crazy paving"); LDH >500 U/L; β-D-glucan >80 pg/mL; hospitalization + adjunctive corticosteroids mandatory |
| Respiratory failure / PCP-ARDS | CD4 <50; delayed diagnosis; PCP-IRIS | Critical | Mechanical ventilation required; mortality 40–60%; pneumothorax (spontaneous) in 10–15% — ominous sign; ICU care |
| PCP in non-HIV immunosuppressed | Solid organ transplant; haematologic malignancy; anti-CD20/CAR-T; high-dose steroids; biologics (anti-TNF) | Severe | More acute and severe onset than AIDS-PCP; less foamy exudate; more neutrophilic inflammation; higher mortality (30–50%); prophylaxis essential |
| Extrapulmonary pneumocystosis | Extremely rare; pentamidine aerosolised prophylaxis | Moderate | Liver, spleen, lymph node involvement; calcified lesions on CT; occurs when pulmonary infection escapes local containment and aerosolised pentamidine provides only pulmonary prophylaxis |
Treatment & Prophylaxis
TMP-SMX and Alternatives
| First-line treatment | TMP 15–20 mg/kg/day + SMX 75–100 mg/kg/day IV or PO in 3–4 divided doses × 21 days; IV preferred for PaO₂ <70 mmHg; oral bioavailability ~100% allows PO switch once improving |
| Adjunctive corticosteroids | Prednisolone 40 mg BID days 1–5, 40 mg/day days 6–10, 20 mg/day days 11–21 (or methylprednisolone IV 75%); indicated when PaO₂ <70 mmHg or A-a gradient >35 mmHg; reduces mechanical ventilation rate and mortality by ~50% |
| Alternative (mild–moderate PCP) | Dapsone 100 mg/day PO + TMP 15 mg/kg/day PO; OR atovaquone 750 mg BID PO (with food) — inferior to TMP-SMX but fewer toxicities; check G6PD before dapsone |
| Alternative (moderate–severe PCP) | Primaquine 30 mg/day base PO + clindamycin 600–900 mg Q8H IV or 450 mg Q6H PO — salvage regimen; comparable efficacy for patients intolerant to TMP-SMX; check G6PD |
| IV pentamidine | 4 mg/kg/day IV — reserved for failure/intolerance; significant toxicity: hypoglycaemia, QT prolongation, nephrotoxicity, pancreatitis; monitor glucose TID |
| Prophylaxis — primary | TMP-SMX DS (160/800 mg) 1 tablet/day or 3×/week — start when CD4 <200/µL (or CD4% <14%); also for ≥20 mg/day prednisone >1 month; organ transplant recipients |
| Prophylaxis — secondary | TMP-SMX DS daily for life or until CD4 >200/µL × 3 months on ART; do NOT discontinue below CD4 200 regardless of viral load |
| Prophylaxis alternatives | Dapsone 100 mg/day; atovaquone 1500 mg/day; aerosolised pentamidine 300 mg/month (nebulised) — inferior, does not prevent extrapulmonary disease |
| Diagnostic workup | BAL (bronchoalveolar lavage) + GMS/DIF stain; serum β-D-glucan >80 pg/mL (sensitivity 95%, specificity 75%); LDH elevated (>500 U/L) — non-specific but correlates with severity; HRCT bilateral GGO ± crazy paving |
| ART in HIV-PCP | Initiate ART within 2 weeks of PCP treatment (unlike cryptococcal meningitis); evidence shows reduced mortality with early ART; PCP-IRIS manageable with continued ART + NSAIDs |
Connections
Alveolar type I pneumocytes (obligate attachment)
Lung interstitium
Liver / spleen (extrapulmonary, rare)
Surfactant SP-A / SP-D function
Gas exchange (foamy intra-alveolar exudate)
Alveolar-capillary barrier
Spontaneous pneumothorax (cyst rupture)
CD4⁺ T cells (gatekeeper)
Dectin-1 / β-glucan axis
DHPS (SMX target)
DHFR (TMP target)
Folate synthesis pathway
References
- Panel on Opportunistic Infections in Adults and Adolescents with HIV. Guidelines for Prevention and Treatment of OIs. NIH/CDC/IDSA 2024. pneumocystis-jirovecii-pneumonia section.
- Kovacs JA & Masur H. Evolving health effects of Pneumocystis. JAMA 2009;301:2578–85.
- Bozzette SA et al. Adjunctive therapy for Pneumocystis pneumonia. N Engl J Med 1990;323:1444–50.
- Tasaka S. Recent advances in the diagnosis and management of Pneumocystis pneumonia. Tuberculosis Respir Dis 2020;83:132–140.
- Krajicek BJ et al. Pneumocystis jiroveci: review of the literature and its role in clinical practice. J Fungi 2022;8:1–18.
- Limper AH et al. An Official American Thoracic Society Statement: Treatment of Fungal Infections in Adult Pulmonary and Critical Care Patients. Am J Respir Crit Care Med 2011;183:96–128.
- Stringer JR et al. A new name for Pneumocystis from humans and new perspectives on the host-pathogen relationship. Emerg Infect Dis 2002;8:891–896.