Kala Azar (Visceral Leishmaniasis) - Immunology

1. Parasite Life Cycle and Entry into the Immune System

2. Innate Immune Evasion - How the Parasite Survives

• Lipophosphoglycan (LPG): Interferes with complement activation pathways
• Gp63 (Leishmanolysin): A zinc-dependent metalloproteinase that cleaves critical host enzymes and inactivates complement components

• Amastigotes maintain phagolysosome pH at 6.5 (instead of normal pH 4.5), protecting themselves from acid hydrolases and reactive oxygen species

• Interfere with phagolysosome formation and granule fusion
• Localize to compartments insusceptible to lytic mechanisms
• Resist reactive oxygen species (ROS) toxicity
• Produce endonucleases that degrade neutrophil extracellular traps (NETs)
• Express protease-resistant surface molecules

• Amastigotes use ZIP family iron transporters to stimulate iron entry into phagolysosomes (iron is essential for parasite survival)
• The host counters with pro-inflammatory cytokines that downregulate transferrin receptors and remove iron from the phagosome

• Parasite-derived miRNAs disrupt host cellular signaling, further compromising immune activation - Robbins, Cotran & Kumar

3. The Central Th1/Th2 Paradigm

Protective Immunity (Th1 Response)

• CD4+ Th1 cells are essential for parasite control
• Th1 cells produce IFN-γ, which activates macrophages via the classical (M1) pathway
• Activated macrophages generate reactive oxygen/nitrogen intermediates (ROI/RNI), particularly nitric oxide (NO) via iNOS, which kills intracellular amastigotes
• In resistant mouse strains, dominant IFN-γ production correlates with parasite clearance

Disease-Promoting Response (Th2 Response)

• Susceptible hosts (and active human disease) show a dominant Th2 response
• Th2 cytokines (IL-4, IL-5, IL-13) inhibit macrophage microbicidal activity
• IL-4 and IL-13 drive alternative macrophage activation (M2), which is permissive to parasite survival
• IL-10 (also produced by regulatory T cells) is a key immunosuppressive cytokine in active VL - it inhibits IFN-γ signaling and macrophage activation
• TGF-β further suppresses effector T cell responses

• Depressed cell-mediated immunity (CMI) - negative leishmanin skin test
• Polyclonal hypergammaglobulinemia - massive B cell activation producing non-protective antibodies
• This antibody response is NOT protective; Leishmania is an intracellular parasite and cannot be killed by antibodies alone

4. Leishmanin (Montenegro) Skin Test - Immunological Significance

5. Humoral Immune Changes in Active Kala Azar

• Polyclonal hypergammaglobulinemia: Massive, non-specific B cell activation
• Elevated IgG, IgM - largely non-protective
• High anti-Leishmania antibody titers (basis of serological tests: rK39 RDT, DAT)
• Hypoalbuminemia from chronic inflammation and liver involvement
• Complement consumption due to immune complex formation
• Immune complex-mediated mesangioproliferative glomerulonephritis can occur in the kidneys; advanced cases may show amyloid deposition

6. Cytokine Profile in Active vs. Cured Disease

7. CD4+ and CD8+ T Cell Roles

• CD4+ Th1 cells: Primary mediators of protection via IFN-γ
• CD8+ CTLs: Can directly kill infected macrophages and produce IFN-γ; contribute to protection but secondary to CD4+
• Regulatory T cells (Tregs): Expanded in active VL; produce IL-10 and TGF-β, suppressing effector responses
• Anergy: T cells in active VL show reduced proliferative responses to Leishmania antigens - restored after treatment

8. The Macrophage as Both Target and Effector

• During active disease: M2 polarization dominates; parasites survive and replicate; upregulation of arginase (metabolizes arginine away from NO production)
• During cure/resistance: M1 activation by IFN-γ and TNF-α; upregulation of iNOS; NO kills amastigotes
• The MPS (mononuclear phagocyte system) is diffusely involved - parasites invade macrophages throughout the spleen, liver (Kupffer cells), bone marrow, lymph nodes, and other organs

9. Pathological Consequences of Immune Dysregulation

• Splenomegaly (up to 3 kg): Hyperplasia of phagocytic cells, loss of normal architecture; plasma cells crowd the pulp
• Hepatomegaly: Kupffer cells packed with amastigotes; late fibrosis
• Pancytopenia: Bone marrow infiltration + hypersplenism
• Hyperpigmentation (the "black fever"/kala-azar): Seen in South Asian patients, related to melanin deposition
• Fever: Cytokine-driven (TNF, IL-1, IL-6)
• Wasting: Chronic inflammation + hypoalbuminemia

10. Post-Kala-Azar Dermal Leishmaniasis (PKDL) - Immune Paradox

• South East Asia: 5-10% of VL cases, appearing 2-10 years after apparent cure
• Sudan: Skin lesions appear in ~50% concurrently with VL

11. HIV-Leishmania Coinfection

• In patients with CD4+ < 200/μL: classic signs (splenomegaly) may be absent
• Higher parasite load; parasites found in atypical sites (gut, lungs)
• Higher sensitivity of microscopy and PCR of blood/bone marrow
• Frequent relapses after treatment - Goldman-Cecil Medicine

12. Immunity After Cure and Vaccination Rationale

• Leishmanin skin test becomes positive - CMI is restored
• IFN-γ responses are reconstituted
• Sterile immunity is NOT achieved - parasites persist in small numbers, preventing reinfection (concomitant immunity)
• This forms the immunological basis for vaccine development: vaccines aim to prime Th1/IFN-γ responses without requiring live infection

Summary Schematic

Sandfly bite → Promastigotes in skin
       ↓
Phagocytosed by macrophages (+ neutrophils)
       ↓
Amastigotes in phagolysosome
       ↓
          ┌──────────────────────────────┐
          │   IMMUNE OUTCOME FORK        │
          └──────────────────────────────┘
   Th1 dominant                   Th2/Treg dominant
   (resistant host)                (susceptible host)
        ↓                                ↓
IFN-γ → M1 macrophage           IL-4, IL-10, TGF-β
iNOS → NO → kills parasite       M2 macrophage
DTH positive (skin test +)       Parasite survival
CURE / IMMUNITY                  ACTIVE KALA AZAR
                                 Leishmanin test (-ve)
                                 Polyclonal IgG ↑↑
                                 Pancytopenia
                                 Hepatosplenomegaly

1. Leishmania is an obligate intracellular parasite of macrophages - cell-mediated immunity (not antibody) is protective
2. Th1 (IFN-γ) = protection; Th2/Treg (IL-4, IL-10, TGF-β) = susceptibility
3. Leishmanin test is negative in active kala azar (anergy), becomes positive after cure
4. Polyclonal hypergammaglobulinemia occurs but is non-protective
5. IL-10 is the dominant immunosuppressive cytokine in active VL
6. Parasite survival strategies: LPG (anti-complement), Gp63, pH manipulation, NET resistance, miRNA interference
7. HIV and T-cell immunodeficiency dramatically worsen disease
8. PKDL is the post-treatment complication due to persistent skin immune dysregulation