Adrian Bot, Stefan Antohi and Constantin Bona

The Department of Microbiology, Mount Sinai Medical School, New York, USA

Received 4/8/97; Accepted 4/15/97; On-line 5/1/97

5. PERSPECTIVE: TOWARDS THE DEVELOPMENT OF A NEW GENETATION OF EFFICIENT VACCINES FOR INFANTS

Lower responsiveness to antigens and high susceptibility to tolerance are the two main characteristics of the immune response of neonates. Earlier and more recent studies showed that in certain conditions, the immunization of newborns led to protective humoral and cellular responses. This is in sharp contrast to the previous paradigm that the neonatal lymphocytes inevitably become tolerant subsequent to the antigen exposure. Recent studies suggested that: (a) the classical 'neonatal tolerance' is in fact a shifted response that comprises mainly the Th2 cell induction; (b) protective responses can be obtained following neonatal immunization by decreasing the dose and prolonging the exposure to antigens, or by co-injecting certain adjuvants.

Our experimental results suggest that naked DNA vaccines can have potential broad benefits as immunization strategy for neonates and young adults. The magnitude of the immune responses obtained by vaccination with plasmids encoding individual proteins of influenza virus, was lower than in that observed with the live-virus immunization. A method to enhance the efficacy of plasmid immunization, is to associate in the same vaccine formulation, multiple epitopes with synergic effects on the magnitude and protectivity of the immune response. In light of the latest developments in the field of neonatal immune responsiveness, this would be a better strategy than to merely increase the dose of antigens. Immunization of newborn mice with a mixture of plasmids encoding NP and HA proteins of influenza virus, confered complete protection upon later challenge with a 100% lethal dose (Table 3 and manuscript in preparation).

^a) Mice were injected according to the schedule depicted in Fig.1, with 50 µg individual plasmid/dose when inoculated separately or 25 µg+25 µg of each when administered in mixtures;

^b) Mice were challenged by aeorsol infection at 5 weeks after the completion of immunization; results were expressed as surviving/total number of mice in a given group;

^c) Since the inoculation of live WSN virus is lethal for BALB/c newborn mice, they were immunized with UV-attenuated WSN virus.

We can explain this synergy in the following way: first, Th epitopes encoded by the HA plasmid enhanced the generation of CTLs and second, neutralizing antibodies, cytokine producing T cells and cross-reactive CTLs against type A influenza viruses participated together in the response to the infection. Thus, neonatal immunization with naked DNA has the ability to induce broad and protective immune responses.

The naked DNA vaccines have important advantages like stability, low-cost and the ability to generate CTL responses in the absence of potential side-effects due to the injection of live-attenuated vaccines. The long persistence of the plasmid associated with continous stimulation of the lymphocytes may lead to good memory responses and Th1 induction. Furthermore, neonatal immunization with naked DNA may bypass the inhibitory effect of the specific antibodies of maternal origin on the generation of broad immune responses following the vaccination with classical vectors. Due to this particular reason combined with direct side-effects of some formulations, most of the present vaccination schedules are carried out later during the postnatal life.

In conclusion, plasmid vaccination may be a potentially useful strategy of neonatal immunization against common pathogens that cause major infectious diseases in newborns and infants, such as Hepatitis B virus, Herpes simplex virus, HIV, Rotaviruses, Plasmodia, Ortho and Paramyxoviruses, etc..