Disorders Associated with Impaired T-Lymphocyte Function (CMI)
Patients with impaired T-lymphocyte/macrophage function have decreased CMI. Disorders
associated with impaired CMI predispose to CAP due to intracellular pathogens, i.e., viruses,
Rickettsiae, systemic mycoses, and intracellular bacteria. Impaired CMI does not, per se,
predispose to bacterial CAP pathogens. However, all compromised hosts may be infected with
the same usual CAP pathogens of normal hosts. Therefore, the intracellular pathogens
associated with severe CAP with decreased CMI are predominantly intracellular pathogens,
i.e.,Pneumocytis (carinii) jiroveci (PCP), cytomegalovirus (CMV), andLegionella. The most
common disorders associated with decreased CMI include chronic/high-dose corticosteroid
therapy, immunosuppressive therapy, TNF-aantagonists, organ transplants, and HIV. As
mentioned previously, CLL is an example of an acquired combined B-/T-lymphocyte defect
with impaired HI greater than CMI (1,8,10,31–35).
Disorders Associated with Impaired B/T-Lymphocyte Function (HI/CMI)
Excluding CLL, most disorders with combined immune defects are those with underlying
B-lymphocyte disorders combined with an immunosuppressant drug which, in addition,
decreases CMI, i.e., inflammatory bowel disease treated with monoclonal antibody therapy,
steroids, or immunosuppressives, etc. The clinician should appreciate the additive effects of
combined immune defects. For example, SLE is a pure B-lymphocyte defect with decreased HI,
but SLE patients with flare resemble CLL with predominantly impaired HI and decreased
CMI. However, SLE patients with flare on corticosteroids/immunosuppressive therapy add to
the “net immunosuppression” further impairing CMI markedly impaired T-lymphocyte
function not unlike that of transplant patients (1,4,17,36,37).
DISORDERS ASSOCIATED WITH DECREASED POLYMORPHONUCLEAR
CELL FUNCTION (PMN)
Chemotherapy is often associated with neutropenia. When the peripheral WBC count is less
than 1 K/mm
3
, the incidence of infection greatly increases. Neutropenia predisposes to
Pseudomonas aeruginosa bacteremia or aerobic gram-negative bacilli (GNB) bacteremias.
Patients with prolonged neutropenia (>1 week) are predisposed to Aspergillus sp.or Candida
sp.However, even though their WBC counts are very low, neutropenic patients present with
bacteremia or fungemia rather thanP. aeruginosaCAP (2,8,10).
Table 7 Nonspecific Laboratory Clues to the Etiology of Severe CAP
Nonspecific laboratory test clues CAP pathogen associations
.Leukopenia Human influenza A, avian influenza (H5N1), swine
influenza (H1N1), SARS, adenovirus, CMV
.Relative lymphopenia PCP, Legionnaire’s disease, adenovirus,
CMV, SARS, human influenzaA,
avian influenza (H5N1), swine influenza, HPS
.Acute thrombocytopenia Adenovirus, CMV, SARS, HPS, human influenza A, avian
influenza (H5N1), swine influenza (H1N1)
.Acute thrombocytosis Q fever,M. pneumoniae
.:::LDH PCP
Avian influenza (H5N1)
.:::Ferritin levels Legionnaire’s disease
.;Serum sodium Legionnaire’s disease
.;Serum phosphorus Legionnaire’s disease
.:Cold agglutinins M. pneumoniae
Adenovirus
Q fever
.:SGOT/SGPT Legionnaire’s disease, Q Fever, CMV, adenovirus, HPS,
SARS, human influenza A, avian influenza (H5N1),
swine influenza (H1N1)Abbreviations: CAP, community-acquired pneumonia, SARS, severe acute respiratory syndrome; PCP,
Pneumocystis (carinii) jiroveciipneumonia; CMV, cytomegalovirus; HPS, hantavirus pulmonary syndrome.
Severe Community-Acquired Pneumonia in Critical Care 169